LeetCode——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.

» Solve this problem

链表操作，注意处理好头节点以及NULL的情况。

/**
 * 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 (head == NULL || head->next == NULL) {
            return head;
        }
        ListNode *p1 = NULL, *p2 = head, *p3 = p2->next, *p4 = p3->next;
        while (p2 != NULL && p3 != NULL) {
            p3->next = p2;
            p2->next = p4;
            if (p1 != NULL) {
                p1->next = p3;
            }
            else {
                head = p3;
            }
            
            p1 = p2;
            p2 = p4;
            if (p2 != NULL) {
                p3 = p2->next;
                if (p3 != NULL) {
                    p4 = p3->next;                                    
                }
            }
        }
        return head;
    }
};