24.Swap Nodes in Pairs

Tags: Medium Linked List

Links: https://leetcode.com/problems/swap-nodes-in-pairs/

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

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

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

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/**
 * 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 == NULL || head -> next == NULL) return head;

        ListNode *dummy = new ListNode(0);
        dummy -> next = head;
        ListNode *pre = dummy -> next, *cur = pre -> next, *head2 = dummy;
        bool flag = false;

        while(pre -> next && cur -> next){
            pre -> next = cur -> next;
            cur -> next = head2 -> next;
            head2 -> next = cur;
            pre = pre -> next;
            cur = pre -> next;
            head2 = head2 -> next -> next;
            if (cur == NULL){
                flag = true;
                break;
            } 
        }
        if (flag) return dummy -> next;
        else{
            pre -> next = cur -> next;
            cur -> next = head2 -> next;
            head2 -> next = cur;
        }

        return dummy -> next;
    }
};

考虑个数为奇数的情形，采用一个flag来记录是否出现奇数情形。方法主要就是交换节点，除了pre, cur外，还需要一个head2来记录pre前面的节点。