LC.P24[两两交换链表中的节点]

方法一：递归

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode newHead = head.next;
        head.next = swapPairs(newHead.next);
        newHead.next = head;
        return newHead;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：迭代

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode ans = new ListNode(0, head);
        ListNode pre = ans, cur = head;
        while (cur != null && cur.next != null) {
            ListNode temp = cur.next;
            cur.next = temp.next;
            temp.next = cur;
            pre.next = temp;
            pre = cur;
            cur = cur.next;
        }
        return ans.next;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(1)$