Wednesday, January 7, 2015

LeetCode 24: 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.
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null)
            return null;
            
        ListNode helper = new ListNode(0);
        helper.next = head;
        ListNode pre = helper;
        ListNode p1 = head;
        ListNode p2 = p1.next;
        
        while (p2 != null)
        {
            pre.next = p2;
            p1.next = p2.next;
            p2.next = p1;
            
            pre = p1;
            p1 = p1.next;
            
            if (p1 != null)
                p2 = p1.next;
            else
                break;
        }
        
        return helper.next;
    }
}

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