有序链表合并

普通版.js

/**
 * 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(0)
    var curr = head
    
    while (l1 !== null && l2 !== null) {
        if (l1.val < l2.val) {
            curr.next = l1
            l1 = l1.next
        } else {
            curr.next = l2
            l2 = l2.next
        }
        curr = curr.next
    }
    
    if (l1 === null) {
        curr.next = l2
    }
        
    if (l2 === null) {
        curr.next = l1
    }
    
    return head.next
};

递归版.js

/**
 * 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 === null && l2 === null) return null
    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
    }
};