21. Merge Two Sorted Lists - jiejackyzhang/leetcode-note GitHub Wiki

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.

Linked List类题目。

采用merge sort的思想。

##Approach 1: iterative

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

##Approach 2: recursive

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        if(l1.val > l2.val) {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        } else {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }
    }
}