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.

比起第一次写简洁多了。

/**
 * 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) return head;
        ListNode h(0), *first = head, *second, *prev = &h, *nextFirst;
        h.next = head;
        
        while (first) {
            second = first->next;
            if (!second) break;
            prev->next = second;
            nextFirst = second->next;
            second->next = first;
            first->next = nextFirst;
            prev = first;
            first = nextFirst;
        }
        
        return h.next;
    }
};