Swap Nodes in Pairs——LeetCode

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.

题目大意：给定一个单链表，交换每两个元素。

解题思路：采用头插法重新构建这个链表，每插入两个就把头指针移动到最后，再插入新的节点。

Talk is cheap>>

    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode res = new ListNode(0);
        res = head.next;
        ListNode cur = new ListNode(0);
        ListNode tmp = head;
        ListNode st = head;
        int cnt = 1;
        while (cur != null && st != null) {
            tmp = st.next;
            st.next = cur.next;
            cur.next = st;
            st = tmp;
            if (cnt == 2) {
                cnt = 0;
                cur = cur.next.next;
            }
            cnt++;
        }
        return res;
    }