[LeetCode-24] Swap Nodes in Pairs

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.

 

嗯，注意指针别转乱了就ok~

 

/**
 * 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) {
            // Start typing your C/C++ solution below
            // DO NOT write int main() function
            if (NULL == head) {
                return NULL;
            }
            ListNode *swap1 = head;
            ListNode *swap2 = head->next;
            ListNode *swapb = NULL;
            ListNode *new_head = (NULL == swap2 ? swap1 : swap2);
            while (NULL != swap1 && NULL != swap2) {
                if (NULL != swapb) {
                    swapb->next = swap2;
                }
                swap1->next = swap2->next;
                swap2->next = swap1;
                swapb = swap1;
                swap1 = swap1->next;
                if (NULL != swap1) {
                    swap2 = swap1->next;
                }
            }
            return new_head;
        }
};