算法-链表-合并两个有序链表

方法一 直接法 一个个比较

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    
    
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    
    
        if(l1 == null) {
    
    
            return l2;
        }
        if(l2 == null) {
    
    
            return l1;
        }
        ListNode head = new ListNode(0);
        ListNode p = head;
        while(l1 != null && l2 != null) {
    
    
            if(l1.val < l2.val) {
    
    
                head.next = l1;
                head = head.next;
                l1 = l1.next;
            }else {
    
    
                head.next = l2;
                head = head.next;
                l2 = l2.next;
            }
        }

        while(l1 != null) {
    
    
            head.next = l1;
            head = head.next;
            l1 = l1.next;
        }
         while(l2 != null) {
    
    
            head.next = l2;
            head = head.next;
            l2 = l2.next;
        }

        return p.next;
    }
}

方法二 递归

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    
    
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    
    

        return help(l1, l2);

    }

    public ListNode help(ListNode l1, ListNode l2) {
    
    
        if(l1 == null) {
    
    
            return l2;
        }

        if(l2 == null) {
    
    
            return l1;
        }

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