[LeetCode 第7题] -- Sort List

题目链接: Sort List

题目意思: 给定一个链表头结点，在O(nlogn)时间内进行排序

分析: 比较排序下限是O(nlogn)，可以选择归并排序解决(事实证明，快速排序会TLE)

代码:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void merge(vector<ListNode*> &arrList, int begin, int mid, int end);
    void mergeSort(vector<ListNode*> &arrList, int begin, int end);
    ListNode *sortList(ListNode *head);
};

void Solution::merge(vector<ListNode*> &arrList, int begin, int mid, int end){
    int i = begin;
    int j = mid+1;
    int pos = 0;
    int* tmpArr = new int[end-begin+2];
        
    while ((i <= mid) && (j <= end)) {
          if (arrList[i]->val <= arrList[j]->val) {
             tmpArr[pos++] = arrList[i++]->val;
          }
          else {
             tmpArr[pos++] = arrList[j++]->val;
          }
    }
    while (i <= mid) {
    	  tmpArr[pos++] = arrList[i++]->val;
    }
    while (j <= end) {
          tmpArr[pos++] = arrList[j++]->val;
    }
    for (i = 0, j = begin; i < pos; i++, j++) {
          arrList[j]->val = tmpArr[i];
    }
    delete tmpArr;
    tmpArr = NULL;
}

void Solution::mergeSort(vector<ListNode*> &arrList, int begin, int end) {
     if (begin >= end) {
         return;
     }
     int mid = (begin+end) >> 1;
     mergeSort(arrList, begin, mid);
     mergeSort(arrList, mid+1, end);
     merge(arrList, begin, mid, end);
}

ListNode* Solution::sortList(ListNode *head) {
     if (NULL == head) {
         return NULL;
     }
     ListNode* tmpHead = head;
     vector<ListNode*> arrList;
     while (tmpHead != NULL) {
         arrList.push_back(tmpHead);
         tmpHead = tmpHead->next;
     }
     mergeSort(arrList, 0, arrList.size()-1);
     return head;
}