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二分法

222. 完全二叉树的节点个数

/*
 * 完全二叉树编号从1开始
 * 如果第k个节点位于第h层，则k的二进制表示包含h+1位，
 * 其中最高位是1，其余各位从高到低表示从根节点到第k个节点的路径，
 * 0表示移动到左子节点，1表示移动到右子节点。
 * 通过位运算得到第k个节点对应的路径，判断该路径对应的节点是否存在，即可判断第k个节点是否存在。
 */
bool exist(struct TreeNode *root, int height, int k) {
    // 树高height（从1开始），从根到叶节点需要往下走height-1次
    int count = height - 1;

    while (count-- > 0) {
        if (root == NULL) break;
        // 从第二位开始，根据每一位判断往左往右
        int bit = ((k >> count) & 1);
        if (bit == 0) {
            root = root->left;
        } else {
            root = root->right;
        }
    }

    // 返回是否存在
    return root != NULL;
}

int binarySearch(struct TreeNode *root, int height, int left, int right) {
    int mid;
    // 找最后一个节点的编号
    while (left <= right) {
        mid = (right - left) / 2 + left;
        if (exist(root, height, mid)) {
            // 若这个编号为mid的节点存在，往右找
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }
    return right;
}

// 完全二叉树的节点个数
int countNodes(struct TreeNode *root) {
    if (root == NULL) return 0;
    struct TreeNode *node = root;
    int height = 0;
    while (node != NULL) {
        height++;
        node = node->left;
    }

    // 最下层，最左边的节点编号
    int left = (1 << (height - 1));
    // 最下层，最右边的节点编号
    int right = (1 << height) - 1;
    return binarySearch(root, height, left, right);
}

2089. 找出数组排序后的目标下标

int cmp(const void *a, const void *b) {
    return (*(int *) a - *(int *) b);
}

// 找左边界
int findLeft(int *nums, int left, int right, int target) {
    int mid;
    while (left <= right) {
        mid = (right - left) / 2 + left;
        if (nums[mid] >= target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return left;
}

// 找右边界
int findRight(int *nums, int left, int right, int target) {
    int mid;
    while (left <= right) {
        mid = (right - left) / 2 + left;
        if (nums[mid] > target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return right;
}

// 排完序后找左右边界
int *targetIndices(int *nums, int numsSize, int target, int *returnSize) {
    qsort(nums, numsSize, sizeof(int), cmp);
    int left = findLeft(nums, 0, numsSize - 1, target);
    int right = findRight(nums, 0, numsSize - 1, target);
    int count = right - left + 1;

    *returnSize = 0;
    if (count <= 0)return NULL;
    int *res = (int *) malloc(sizeof(int) * count);
    for (int i = 0; i < count; ++i) {
        res[(*returnSize)++] = left + i;
    }
    return res;
}

2529. 正整数和负整数的最大计数

int countPos(int *array, int start, int end) {
    // 全是非正数
    if (array[end] <= 0) return 0;
    // 左右边界
    int i;
    int j = end;

    int left = start, right = end;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] > 0)
            right = mid - 1;
        else
            left = mid + 1;
    }
    i = left;
    return j - i + 1;
}

int countNeg(int *array, int start, int end) {
    // 全是非负数
    if (array[start] >= 0) return 0;
    // 左右边界
    int i = start;
    int j;

    int left = start, right = end;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] >= 0)
            right = mid - 1;
        else
            left = mid + 1;
    }
    j = right;
    return j - i + 1;
}

int maximumCount(int *nums, int numsSize) {
    int i = countPos(nums, 0, numsSize - 1);
    int j = countNeg(nums, 0, numsSize - 1);
    return i > j ? i : j;
}

1351. 统计有序矩阵中的负数

// 在非递增序列中找-1插入的左边界
int findPosition(int *array, int left, int right) {
    int mid;
    int target = -1;
    while (left <= right) {
        mid = (right - left) / 2 + left;
        if (array[mid] <= target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return left;
}

// 一行行统计，没有利用到列也是非递增的条件
int countNegatives(int **grid, int gridSize, int *gridColSize) {
    int res = 0;
    for (int i = 0; i < gridSize; ++i) {
        res += gridColSize[i] - findPosition(grid[i], 0, gridColSize[i] - 1);
    }
    return res;
}

// 在非递增序列中找-1插入的左边界
int findPosition(int *array, int left, int right) {
    int mid;
    int target = -1;
    while (left <= right) {
        mid = (right - left) / 2 + left;
        if (array[mid] <= target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return left;
}

// 每一行从前往后第一个负数的位置是不断递减的
// 右边界也就是-1插入的位置，也就是负数应该出现的位置，下标不断减小。没必要每次都以gridSize作为二分法右边界
int countNegatives(int **grid, int gridSize, int *gridColSize) {
    int res = 0;

    int tempPos = gridColSize[0];
    int temp;
    // 处理每行
    for (int i = 0; i < gridSize; ++i) {
        // 下标从0到第一个负数出现的位置
        temp = findPosition(grid[i], 0, tempPos - 1);
        res += gridColSize[i] - temp;
        // 更新第一个负数的下标
        tempPos = temp;
    }
    return res;
}

2389. 和有限的最长子序列

int cmp(const void *a, const void *b) {
    return (*(int *) a - *(int *) b);
}

// 找右边界
int binarySearch(int *array, int left, int right, int target) {
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] <= target)
            left = mid + 1;
        else
            right = mid - 1;
    }
    return right;
}

int *answerQueries(int *nums, int numsSize, int *queries, int queriesSize, int *returnSize) {
    int *res = (int *) malloc(sizeof(int) * queriesSize);
    *returnSize = queriesSize;
    qsort(nums, numsSize, sizeof(int), cmp);

    // 保存前缀和
    int sum[numsSize];
    sum[0] = nums[0];
    for (int i = 1; i < numsSize; ++i) {
        sum[i] = sum[i - 1] + nums[i];
    }

    // 在前缀和数组里二分查找
    for (int i = 0; i < queriesSize; ++i) {
        res[i] = binarySearch(sum, 0, numsSize - 1, queries[i]) + 1;
    }
    return res;
}

LCR 069. 山脉数组的峰顶索引

// todo
int peakIndexInMountainArray(int *arr, int arrSize) {
    int left = 0;
    int right = arrSize - 1;
    int mid;
    while (left < right) {
        mid = left + (right - left) / 2;
        if (arr[mid] < arr[mid + 1]) {
            // left左边全都小于arr[left], arr[left]=arr[mid + 1]最大
            left = mid + 1;
        } else if (arr[mid] > arr[mid + 1]) {
            // right右边全都小于arr[right]，arr[right]=arr[mid]最大
            right = mid;
        }
    }
    return left;
}

1337. 矩阵中战斗力最弱的 K 行

// todo

LCR 179. 查找总价格为目标值的两个商品

// 双指针
int *twoSum(int *numbers, int numbersSize, int target, int *returnSize) {
    int *res = (int *) malloc(sizeof(int) * 2);
    *returnSize = 0;

    int left = 0, right = numbersSize - 1;

    while (left < right) {
        if (numbers[left] == target - numbers[right]) {
            res[0] = numbers[left];
            res[1] = numbers[right];
            *returnSize = 2;
            return res;
        } else if (numbers[left] > target - numbers[right]) {
            right--;
        } else {
            left++;
        }
    }
    return res;
}

// 散列
int *twoSum(int *numbers, int numbersSize, int target, int *returnSize) {
    int *res = (int *) malloc(sizeof(int) * 2);
    *returnSize = 0;

    int *hashMap = (int *) calloc(2000001, sizeof(int));
    for (int i = 0; i < numbersSize; ++i) {
        if ((target - numbers[i]) > 0 && hashMap[target - numbers[i]] == 1) {
            res[0] = numbers[i];
            res[1] = target - numbers[i];
            *returnSize = 2;
            return res;
        }
        hashMap[numbers[i]] = 1;
    }
    return res;
}

面试题 08.03. 魔术索引

// todo
// 二分+递归
int binarySearch(int *array, int left, int right) {
    if (left > right) return -1;
    int mid = ((right - left) >> 1) + left;
    // 左区间能找到就返回左区间
    int leftAns = binarySearch(array, left, mid - 1);
    if (leftAns != -1) return leftAns;
    // 左区间找不到就检查当前节点
    if (array[mid] == mid) return mid;
    // 当前节点也不满足就检查右区间
    return binarySearch(array, mid + 1, right);
}

int findMagicIndex(int *nums, int numsSize) {
    return binarySearch(nums, 0, numsSize - 1);
}

LCR 006. 两数之和 II - 输入有序数组

// 双指针
// 假设数组中存在且只存在一对符合条件的数字，同时一个数字不能使用两次
int *twoSum(int *numbers, int numbersSize, int target, int *returnSize) {
    int *res = (int *) malloc(sizeof(int) * 2);
    *returnSize = 0;

    int left = 0, right = numbersSize - 1;

    while (left < right) {
        if (numbers[left] == target - numbers[right]) {
            res[0] = left;
            res[1] = right;
            *returnSize = 2;
            return res;
        } else if (numbers[left] > target - numbers[right]) {
            right--;
        } else {
            left++;
        }
    }
    return res;
}

int binarySearch(int *array, int left, int right, int target) {
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] == target) {
            return mid;
        } else if (array[mid] > target) {
            right = mid - 1;
        } else {
            left = mid + 1;
        }
    }
    return -1;
}

// 假设数组中存在且只存在一对符合条件的数字，同时一个数字不能使用两次
// 在当前节点的右边二分查找
int *twoSum(int *numbers, int numbersSize, int target, int *returnSize) {
    int *res = (int *) malloc(sizeof(int) * 2);
    *returnSize = 0;

    for (int i = 0; i < numbersSize; ++i) {
        int k = binarySearch(numbers, i + 1, numbersSize - 1, target - numbers[i]);
        if (k != -1) {
            res[0] = i;
            res[1] = k;
            *returnSize = 2;
            return res;
        }
    }
    return res;
}

1385. 两个数组间的距离值


int cmp(const void *a, const void *b) {
    return (*(int *) a - *(int *) b);
}

// 大于等于（找应该插入的位置，左边界）
int binarySearch1(int *array, int size, int target) {
    int left = 0;
    int right = size - 1;
    int mid;
    while (left <= right) {
        mid = left + (right - left) / 2;
        if (target > array[mid])
            left = mid + 1;     // left更新之后，left左边必然都小于target
        else if (target <= array[mid])
            right = mid - 1;    // right更新之后，right右边必然都大于等于target
    }
    // 循环结束时left = right + 1
    return left;
}

// 小于等于(右边界)
int binarySearch2(int *array, int size, int target) {
    int left = 0;
    int right = size - 1;
    int mid;
    while (left <= right) {
        mid = left + (right - left) / 2;
        if (target >= array[mid])  // 即使等于left也右移，这样小于等于的就都在left左边了，循环结束时，right就在left-1的位置
            left = mid + 1;     // left更新之后，left左边必然都小于等于target
        else if (target < array[mid])
            right = mid - 1;    // right更新之后，right右边必然都大于target
    }
    return right;
}

// 求arr1中与arr2所有元素之差绝对值大于d的元素个数
int findTheDistanceValue(int *arr1, int arr1Size, int *arr2, int arr2Size, int d) {
    qsort(arr2, arr2Size, sizeof(int), cmp);
    int sum = 0;
    for (int i = 0; i < arr1Size; ++i) {
        int right = binarySearch1(arr2, arr2Size, arr1[i]);
        int left = binarySearch2(arr2, arr2Size, arr1[i]);
        printf("%d %d\n", left, right);
        if ((left < 0 || abs(arr2[left] - arr1[i]) > d)
            && (right >= arr2Size || abs(arr2[right] - arr1[i]) > d)) {

            sum++;

        }
    }
    return sum;
}

888. 公平的糖果交换

int cmp(const void *a, const void *b) {
    return *(int *) a - *(int *) b;
}

int binarySearch(int *array, int size, int target) {
    int left = 0;
    int right = size - 1;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (target == array[mid])
            return mid;
        else if (target < array[mid])
            right = mid - 1;
        else
            left = mid + 1;
    }
    return -1;
}

int *fairCandySwap(int *aliceSizes, int aliceSizesSize, int *bobSizes, int bobSizesSize, int *returnSize) {
    int *res = (int *) malloc(sizeof(int) * 2);
    *returnSize = 2;
    
    // 先把bobSizes排序，再根据aliceSizes[i]在bobSizes中二分查找
    qsort(bobSizes, bobSizesSize, sizeof(int), cmp);
    int sum1 = 0, sum2 = 0;
    for (int i = 0; i < aliceSizesSize; ++i) sum1 += aliceSizes[i];
    for (int i = 0; i < bobSizesSize; ++i) sum2 += bobSizes[i];
    // sum1给出x，sum2给出y
    // sum1-x+y = sum2-y+x
    // gap = sum1-sum2 = 2*(x-y)
    // y = x - gap/2
    int gap = sum1 - sum2;

    for (int i = 0; i < aliceSizesSize; ++i) {
        int t = binarySearch(bobSizes, bobSizesSize, aliceSizes[i] - gap / 2);
        if (t != -1) {
            res[0] = aliceSizes[i];
            res[1] = bobSizes[t];
            return res;
        }
    }
    return res;
}

1608. 特殊数组的特征值

int cmp(const void *a, const void *b) {
    return (*(int *) a) - (*(int *) b);
}

// 左边界
int binarySearch(int *array, int size, int target) {
    int left = 0;
    int right = size - 1;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] >= target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return left;
}

int specialArray(int *nums, int numsSize) {
    qsort(nums, numsSize, sizeof(int), cmp);
    for (int i = 0; i <= nums[numsSize - 1]; ++i) {
        int index = binarySearch(nums, numsSize, i);
        if (numsSize - index == i)
            return i;
    }
    return -1;
}

350. 两个数组的交集 II

// 散列
int *intersect(int *nums1, int nums1Size, int *nums2, int nums2Size, int *returnSize) {
    int min = nums1Size > nums2Size ? nums2Size : nums1Size;
    int *res = (int *) malloc(sizeof(int) * min);
    *returnSize = 0;
    int *hashMap = (int *) calloc(1001, sizeof(int));

    // 统计出现次数
    for (int i = 0; i < nums1Size; ++i) hashMap[nums1[i]]++;
    for (int i = 0; i < nums2Size; ++i) {
        if (hashMap[nums2[i]] > 0) {
            // 表中减一
            hashMap[nums2[i]]--;
            res[(*returnSize)++] = nums2[i];
        }
    }
    return res;
}

// 左边界
int binarySearch(int *array, int left, int right, int target) {
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] >= target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return left;
}

int cmp(const void *a, const void *b) {
    return *(int *) a - *(int *) b;
}

// 排序后双指针，第二个指针用二分法定位
int *intersect(int *nums1, int nums1Size, int *nums2, int nums2Size, int *returnSize) {
    int min = nums1Size > nums2Size ? nums2Size : nums1Size;
    int *res = (int *) malloc(sizeof(int) * min);
    *returnSize = 0;

    // 排序
    qsort(nums1, nums1Size, sizeof(int), cmp);
    qsort(nums2, nums2Size, sizeof(int), cmp);

    int j = 0;
    for (int i = 0; i < nums1Size && j < nums2Size; ++i) {
        int k = binarySearch(nums2, j, nums2Size - 1, nums1[i]);
        if (k >= 0 && k < nums2Size && nums1[i] == nums2[k]) {
            res[(*returnSize)++] = nums1[i];
            // 数组2的j指针同时后移
            j = k + 1;
        }
    }
    return res;
}

面试题 10.05. 稀疏数组搜索

int binarySearch(char **words, int left, int right, char *s) {
    int mid;
    while (left <= right) {
        // 跳过左右的空字符串
        while (left <= right && strcmp(words[left], "") == 0) left++;
        while (left <= right && strcmp(words[right], "") == 0) right--;

        mid = ((right - left) >> 1) + left;
        // 中间的是空字符串，就在两侧找
        if (strcmp(words[mid], "") == 0) {
            // 找左边
            int leftAns = binarySearch(words, left, mid - 1, s);
            if (leftAns != -1) return leftAns;
            // 找右边
            return binarySearch(words, mid + 1, right, s);
        }

        // 中间不是空字符串
        int k = strcmp(words[mid], s);
        if (k == 0) {
            return mid;
        } else if (k > 0) {
            right = mid - 1;
        } else if (k < 0) {
            left = mid + 1;
        }
    }

    return -1;
}

// 二分+递归
int findString(char **words, int wordsSize, char *s) {
    return binarySearch(words, 0, wordsSize - 1, s);
}

int findString(char **words, int wordsSize, char *s) {
    int left = 0, right = wordsSize - 1;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        int beforeMid = mid;

        // 无法二分时，线性探测
        while (mid <= right && strcmp(words[mid], "") == 0) {
            mid++;
        }

        if (mid <= right) {
            // 没有越界，words[mid]一定不是空字符串
            if (strcmp(words[mid], s) == 0) {
                return mid;
            } else if (strcmp(words[mid], s) > 0) {
                right = beforeMid - 1;
            } else {
                left = mid + 1;
            }
        } else {
            right = beforeMid - 1;
        }
    }
    return -1;
}

1539. 第 k 个缺失的正整数


int findKthPositive(int *arr, int arrSize, int k) {
    int left = 0, right = arrSize - 1;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        // 到arr[mid]为止，包括arr[mid]，缺失的正整数个数
        int lossCount = arr[mid] - mid - 1;
        // 找左边界
        if (lossCount >= k) {
            // 代码a，执行这条代码，right右面的必然都满足lossCount >= k
            right = mid - 1;
        } else {
            left = mid + 1;
        }
    }
    // 循环结束一定是left=right+1
    // right是刚好lossCount严格小于k的位置，下个位置right+1是lossCount大于等于k的位置（代码a决定的）
    if (right >= 0) {
        
        // index        = [0,1,2,3,4]
        // array        = [2,3,4,7,11]
        // lossCount    = [1,1,1,3,6]
        // right = 3, arr[right] = 7, lossCount[right] = 3, lossCount[right+1] = 6
        // lossCount[right] < k < lossCount[right+1]，丢失的数一定就在arr[right]到arr[right+1]之间

        // 到arr[right]为止，包括arr[right]，缺失的正整数个数为lossCount，从arr[right]往后数（k-lossCount)个数就是答案
        return k - (arr[right] - (right) - 1) + arr[right];
    }
    return k;
}

LCR 172. 统计目标成绩的出现次数

// 左边界
int binarySearch1(int *array, int size, int target) {
    int left = 0, right = size - 1;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] >= target)
            right = mid - 1;
        else
            left = mid + 1;
    }
    return left;
}

// 右边界
int binarySearch2(int *array, int size, int target) {
    int left = 0, right = size - 1;
    int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        if (array[mid] <= target)
            left = mid + 1;
        else
            right = mid - 1;
    }
    return right;
}

int countTarget(int *scores, int scoresSize, int target) {
    int left = binarySearch1(scores, scoresSize, target);
    int right = binarySearch2(scores, scoresSize, target);
    if(left >= scoresSize || scores[left] != target) return 0;
    return right - left + 1;
}

154. 寻找旋转排序数组中的最小值 II

// todo
// 含重复元素的增序数组，循环右移后找最小元素
int findMin(int *nums, int numsSize) {
    int left = 0;
    int right = numsSize - 1;
    int mid;
    // 规律：最小值下标x，[0,x)值都大于等于末尾元素，[x,array.length-1]都小于等于末尾元素
    while (left < right) {
        mid = left + (right - left) / 2;
        // 和末尾元素比较
        if (nums[mid] > nums[right]) {
            // [left, mid]都大于numbers[right]，都排除
            left = mid + 1;
        } else if (nums[mid] < nums[right]) {
            // numbers[mid]是[mid,right]上最小的，忽略(mid,right]上的
            right = mid;
        } else {
            // 忽略末尾，新的末尾numbers[right-1]也符合规律
            right--;
        }
    }
    return nums[left];
}

441. 排列硬币

// 右边界
int arrangeCoins(int n) {
    int left = 1, right = n;
    long int mid;
    // k行全满，共(1+k)*k/2个元素
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        long int sum = (1 + mid) * mid >> 1;
        if (sum <= n)
            left = mid + 1;
        else
            right = mid - 1;
    }
    return right;
}

367. 有效的完全平方数

// 小于等于（右边界
bool isPerfectSquare(int num) {
    int left = 0;
    int right = num;
    long int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        long int temp = mid * mid;
        if (temp <= num)
            left = mid + 1;
        else
            right = mid - 1;
    }
    return right * right == num;
}

LCR 072. x 的平方根

// 小于等于（右边界
int mySqrt(int x) {
    int left = 0;
    int right = x;
    long int mid;
    while (left <= right) {
        mid = ((right - left) >> 1) + left;
        long int temp = mid * mid;
        if (temp <= x)
            left = mid + 1;
        else
            right = mid - 1;
    }
    return right;
}

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