LeetCode_21_Merge Two Sorted Lists

题目描述

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

代码实现（两种方式）

• 第一种：使用非递归的形式

主要思想就是：建立一个头结点，然后依次比较两个链表节点的大小，把小的依次节点挂在头结点后面

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null)
            return l2;
        if (l2 == null)
            return l1;
        ListNode FakeHead = new ListNode();
        ListNode currentNode = FakeHead;
        while(l1 != null && l2 != null) {
            ListNode temp;
            if (l1.val < l2.val) {
                temp = l1;
                l1 = l1.next;
            } else {
                temp = l2;
                l2 = l2.next;
            }
            currentNode.next = temp;
            currentNode = currentNode.next;
        }
        while (l1 != null) {
            currentNode.next = l1;
            currentNode = currentNode.next;
            l1 = l1.next;
        }
        while (l2 != null) {
            currentNode.next = l2;
            currentNode = currentNode.next;
            l2 = l2.next;
        }
    return FakeHead.next;
    }
}
           

• 第二种：递归的形式

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode 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(l2.next, l1);
            return l2;
        }
    }
}