LeetCode 21. 合并两个有序链表(Merge Two Sorted Lists)

21. 合并两个有序链表

21. Merge Two Sorted Lists

题目描述

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

LeetCode21. Merge Two Sorted Lists

示例:

输入: 1->2->4, 1->3->4
输出: 1->1->2->3->4->4

Java 实现

ListNode 类

class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }

    @Override
    public String toString() {
        return val + "->" + next;
    }
}

Iterative Solution

public class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null && list2 == null) {
            return null;
        }
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }
        ListNode head = new ListNode(-1);
        ListNode curr = head;
        while (list1 != null && list2 != null) {
            if (list1.val <= list2.val) {
                curr.next = list1;
                list1 = list1.next;
            } else {
                curr.next = list2;
                list2 = list2.next;
            }
            curr = curr.next;
        }
        if (list1 != null) {
            curr.next = list1;
        }
        if (list2 != null) {
            curr.next = list2;
        }
        return head.next;
    }
}

Recursive Solution

public class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }
        if (list1.val <= list2.val) {
            list1.next = mergeTwoLists(list1.next, list2);
            return list1;
        } else {
            list2.next = mergeTwoLists(list1, list2.next);
            return list2;
        }
    }
}

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参考资料

• https://leetcode.com/problems/merge-two-sorted-lists/
• https://leetcode-cn.com/problems/merge-two-sorted-lists/
• https://leetcode.com/problems/merge-two-sorted-lists/discuss/9715/Java-1-ms-4-lines-codes-using-recursion
• https://leetcode.com/problems/merge-two-sorted-lists/discuss/9858/Java-solution-for-reference