21. Merge Two Sorted Lists

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

JavaScript

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
  var head = new ListNode()
  var current = head

  while (l1 && l2) {
    if (l1.val <= l2.val) {
      current.next = l1
      l1 = l1.next
    } else {
      current.next = l2
      l2 = l2.next
    }
    current = current.next
  }

  current.next = l1 ? l1 : l2
  return head.next
}

JavaScript

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
  if (!l1) {
    return l2
  }
  if (!l2) {
    return l1
  }
  if (l1.val > l2.val) {
    let temp = l1
    l1 = l2
    l2 = temp
  }
  l1.next = mergeTwoLists(l1.next, l2)
  return l1
}

Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None


class Solution(object):

    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        head = ListNode(None)
        temp = head

        while l1 and l2:
            if l1.val <= l2.val:
                temp.next = l1
                l1 = l1.next
            else:
                temp.next = l2
                l2 = l2.next
            temp = temp.next

        temp.next = l1 if l1 else l2

        return head.next

Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None


class Solution(object):

    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1:
            return l2
        if not l2:
            return l1
        if l1.val > l2.val:
            l1, l2 = l2, l1
        l1.next = self.mergeTwoLists(l1.next, l2)
        return l1