算法刷题Day 25 组合总和III+电话号码的字母组合-Toy模板网

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Day 25 回溯算法

216. 组合总和III

class Solution {
    vector<vector<int>> rst;
    vector<int> path;
    int sum = 0;

    void backtracking(int k, int n, int cur)
    {
        if (path.size() == k && sum == n)
        {
            rst.push_back(path);
            return;
        }

        for (int i = cur; i < 10; ++i)
        {
            path.push_back(i);
            sum += i;
            backtracking(k, n, i + 1);
            sum -= i;
            path.pop_back();
        }
    }

public:
    vector<vector<int>> combinationSum3(int k, int n) {
        rst.clear();
        path.clear();
        sum = 0;
        backtracking(k, n, 1);
        return rst;
    }
};

17. 电话号码的字母组合

class Solution {
    vector<string> rst;
    string path;
    const unordered_map<char, string> table = {
        {'2', "abc"},
        {'3', "def"},
        {'4', "ghi"},
        {'5', "jkl"},
        {'6', "mno"},
        {'7', "pqrs"},
        {'8', "tuv"},
        {'9', "wxyz"}
    };

    void backtracking(const string &digits, int idx)
    {
        if (path.size() == digits.size())
        {
            rst.push_back(path);
            return;
        }

        for (int i = 0; i < table.at(digits[idx]).size(); ++i)
        {
            path.push_back(table.at(digits[idx])[i]);
            backtracking(digits, idx + 1);
            path.pop_back();
        }
    }

public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty()) return {};
        rst.clear();
        path = "";
        backtracking(digits, 0);
        return rst;
    }
};