Question

link

Stats

Frequency	5
Diffficulty	2
Adjusted Difficulty	2
Time to use	----------

Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)

Analysis

This question is easy. There are difficult ways to solve.

Solution

There are 2 ways to solve this problem. First and the easy one is to do recursion.

Second solution, also my original solution is to use a ‘fakeHead’ to help. Read this blog.

My code

Recursion:

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

Temporary header + link operations

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