LeetCode之旅（24）

Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

再一次熟悉链表的指针操作。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        if (!head || !head->next) return head;
        ListNode *cur=head->next, *pre=head, *ppre=NULL;
        head = cur;
        while (cur) {
            pre->next = cur->next;
            cur->next = pre;
            if (ppre) ppre->next = cur;
            cur = pre->next;
            if (cur) cur = cur->next;
            ppre = pre;
            pre = pre->next;
        }
        return head;
    }
};