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.

遍历索引，0,1,2,3…索引为偶数就和后面一个把值换了

package pack;

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null) {