C实现 LeetCode->Merge Two Sorted Lists （双指针大法）

Sort a linked list in O(n log n)
time using constant space complexity.

 *   将单链表
排序   O(n lg n)
  



//
// SortList.c
// Algorithms
//
// Created by TTc on 15/6/18.
// Copyright (c) 2015年 TTc. All rights reserved.
//

/**
* Sort a linked list in O(n log n) time using constant space complexity.
* 将单链表 排序
*/
#include "SortList.h"

struct ListNode {
int val;
struct ListNode *next;
};

/**
* 双指针大法,创建两个 指针（首尾指针 向中间扫描）
*/
struct ListNode*
sortList(struct ListNode* head) {
if(!head) return NULL;

struct ListNode *start = head;
struct ListNode *end = head;

int length = 0;
while(start) {
length++;
start = start->next;
}

if(length == 1) return head;

int mid = length/2; //计算结点个数，将链表分开。

//将链表分开
struct ListNode *tmpNode = NULL;

for(int i = 0; i < mid && end != NULL; i++) {
if(i == mid - 1) tmpNode = end;
end = end->next;
}

if(tmpNode != NULL) tmpNode->next = NULL;

//分别对两段排序
start = head;
start = sortList(start);
end = sortList(end);

struct ListNode *newhead = NULL;//新创建的list头节点
struct ListNode *newtemp = NULL;//中间遍历接收变量

while(start && end) { //合并
if(start->val <= end->val) {
if(!newhead) newtemp = newhead = start;
else{
newtemp->next = start;
newtemp = start;
}
start = start->next;
}else{
if(!newhead) newhead = newtemp = end;
else{
newtemp->next = end;
newtemp = end;
}
end = end->next;
}
}
if(start) newtemp->next = start;
if(end) newtemp->next = end;
return newhead;
}