题目:
示例:
思路:
这题我们将使用栈解决这个问题,利用栈先进后出的特点,从链表的中间位置进行入栈,寻找链表的中间位置参考:删除链表的中间节点,之后从头开始进行连接。
本题使用的栈源代码在此处:栈和队列的实现
图示:
代码:
//栈
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <stdbool.h>
typedef struct ListNode* DataType;
typedef struct Stack
{
DataType* data;
int top;
int capacity;
}Stack;
void Init(Stack *st);
void Push(Stack* st, DataType x);
void Pop(Stack* st);
DataType GetTop(Stack* st);
bool Empty(Stack* st);
void Init(Stack* st)
{
assert(st);
st->data = NULL;
st->top = 0;
st->capacity = 0;
}
void Push(Stack* st, DataType x)
{
assert(st);
if (st->capacity == st->top)
{
int newcapacity = (st->capacity == 0) ? 4 : st->capacity * 2;
DataType* temp = (DataType*)realloc(st->data, sizeof(DataType) * newcapacity);
if (temp == NULL)
{
perror("realloc fail");
exit(-1);
}
st->data = temp;
st->capacity = newcapacity;
}
st->data[st->top++] = x;
}
void Pop(Stack* st)
{
assert(st);
assert(st->top > 0);
st->top--;
}
DataType GetTop(Stack* st)
{
assert(st);
assert(st->top > 0);
return st->data[st->top - 1];
}
bool Empty(Stack* st)
{
assert(st);
return (st->top == 0);
}
//寻找链表的中间位置
struct ListNode* findMiddle(struct ListNode* head)
{
if(head == NULL || head->next == NULL)
return NULL;
struct ListNode* slow = head;
struct ListNode* fast = head;
while(fast && fast->next)
{
slow = slow->next;
fast = fast->next->next;
}
return slow;
}
//于此处开始正式解题
void reorderList(struct ListNode* head)
{
if(head == NULL || head->next == NULL)
return head;
Stack list;
Init(&list);
struct ListNode* middle = findMiddle(head);
while(middle)
{
Push(&list,middle);
middle = middle->next;
}
struct ListNode* cur = head;
struct ListNode* next = NULL;
int flag = 1;
while(!Empty(&list))
{
if(flag == 1)
{
next = cur->next;
cur->next = GetTop(&list);
Pop(&list);
flag = 0;
}
else
{
cur->next = next;
flag = 1;
}
cur = cur->next;
}
cur->next = NULL;
return head;
}
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