算法-leetcode-每日一题-合并两个有序链表

合并两个有序链表
Example:
Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

   public class ListNode {
       int val;
       ListNode next;
       ListNode(int x) { val = x; }
   }

分析：该题较为常见，常见的想法就是用归并排序。

public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        ListNode result;
        if (l1 == null && l2 == null) {
            result = null;
        } else if (l1 != null && l2 == null) {
            result = l1;
        } else if (l1 == null && l2 != null) {
            result = l2;
        } else {
            // 归并排序
            if (l1.val <= l2.val) {
                result = l1;
                l1 = l1.next;
            } else {
                result = l2;
                l2 = l2.next;
            }
            ListNode result_end = result;
            while (l1 != null && l2 != null) {
                if (l1.val <= l2.val) {
                    result_end.next = l1;
                    l1 = l1.next;
                } else {
                    result_end.next = l2;
                    l2 = l2.next;
                }
                result_end = result_end.next;
            }
            if (l1 == null) {
                result_end.next = l2;
            } else {
                result_end.next = l1;
            }
        }
        return result;
    }

还有一种是递归方法：

public ListNode mergeTwoLists1(ListNode l1, ListNode l2) {
    if(l1 == null) return l2;
    if(l2 == null) return l1;
    if(l1.val < l2.val){
        l1.next = mergeTwoLists1(l1.next, l2);
        return l1;
    } else{
        l2.next = mergeTwoLists1(l1, l2.next);
        return l2;
    }
}