solved 'Sort List' on LeetCode http://oj.leetcode.com/problems/sort-list/

sort_list.cpp

/* Merge Sort for linked list is very effective */

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
        int size = 0;
        for (ListNode *i = head; i != NULL; i = i->next) size++;
        if (size <= 1) return head;
        
        ListNode *left = head, *right;
        for (int i=0; i<(size-1)/2; ++i) 
            left = left->next;
        
        right      = left->next;
        left->next = NULL;
        left       = sortList(head);
        right      = sortList(right);
        return merge(left, right);
    }
    
    ListNode *merge(ListNode *left, ListNode *right) {
        ListNode *start, *iter;
        if (left->val < right->val) {
            start = left;
            left  = left->next;
        }
        else {
            start = right;
            right = right->next;
        }
        iter = start;
        while (left != NULL && right != NULL) {
            if (left->val < right->val) {
                iter->next = left;
                left       = left->next;
                iter       = iter->next;
            }
            else {
                iter->next = right;
                right      = right->next;
                iter       = iter->next;
            }
        }
        // final link
        if (left == NULL) iter->next = right;
        if (right == NULL) iter->next = left;
        
        return start;
    }
};