leetcode-21-合并两个有序链表

// https://leetcode-cn.com/problems/merge-two-sorted-lists/
/**

• @param {ListNode} l1

• @param {ListNode} l2

• @return {ListNode}
  */
  // 方法一：递归
  var mergeTwoLists = function(l1, l2) {
  if (l1 === null) return l2;
  if (l2 === null) return l1;

  if (l1.val < l2.val) {
  l1.next = mergeTwoLists(l1.next, l2);
  return l1;
  } else {
  l2.next = mergeTwoLists(l1, l2.next);
  return l2;
  }
  };

// 方法二：迭代
var mergeTwoLists = function(l1, l2) {
var preHead = new ListNode(-1);
var prev = preHead;

while(l1 !== null && l2 !== null) {
if (l1.val <= l2.val) {
prev.next = l1;
prev = l1;
l1 = l1.next;
} else {
prev.next = l2;
prev = l2;
l2 = l2.next;
}
}
// 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可
if (l1 !== null) {
prev.next = l1;
} else if (l2 !== null) {
prev.next = l2;
}
return preHead.next;
}