21.合并两个有序链表

/*
 * @lc app=leetcode.cn id=21 lang=typescript
 *
 * [21] 合并两个有序链表
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

class ListNode {
	val: number;
	next: ListNode | null;
	constructor(val?: number, next?: ListNode | null) {
		this.val = val === undefined ? 0 : val;
		this.next = next === undefined ? null : next;
	}
}

function mergeTwoLists(
	list1: ListNode | null,
	list2: ListNode | null
): ListNode | null {
	if (!list1) return list2;
	if (!list2) return list1;

	if (list1.val < list2.val) {
		const rst = mergeTwoLists(list1.next, list2);
		list1.next = rst;
		return list1;
	} else {
		const rst = mergeTwoLists(list1, list2.next);
		list2.next = rst;
		return list2;
	}
}
// @lc code=end