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  • [LeetCode 024] Swap Nodes in Pairs

    Question

    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.

    Codes

    Iterative version

    public class Solution {
        public ListNode swapPairs(ListNode head) {
            if (head == null) {
                return null;
            }
            ListNode anchor = new ListNode(0); // add a new node pointing to head
            anchor.next = head;
            ListNode pre = anchor; // the node before the pair
            ListNode next; // the node which the second node in pair points to
            ListNode node1; // the first node in pair
            while (pre.next != null && pre.next.next != null) {
                node1 = pre.next; // initial the first node
                next = pre.next.next.next; 
                node1.next.next = node1; // node1.next is the second node; set the second node pointing to the first node
                pre.next = node1.next; // set the node before the pair pointing to the second node 
                node1.next = next; // set the first node pointing to next node
                pre = pre.next.next; // initial pre node
            }
            return anchor.next;
        }
    }
    

    Recursive version

    public class Solution {
        public ListNode swapPairs(ListNode head) {
            if (head == null || head.next == null) {
                return head;
            }
    
            ListNode newPair = head.next.next;
            ListNode newHead = head.next;
            head.next.next = head;
            head.next = swapPairs(newPair);
            return newHead;
        }
    }
    
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  • 原文地址:https://www.cnblogs.com/Victor-Han/p/5157796.html
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