/*
 * @lc app=leetcode id=21 lang=java
 *
 * [21] Merge Two Sorted Lists
 */

// @lc code=start
/**
 * 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 list1, ListNode list2) {
        if(list1 == null || list2 == null) {
            return list1 == null?list2:list1;
        }
        ListNode h = null, cur = null;
        while(list1 != null && list2!=null) {
            if(list1.val <= list2.val) {
                if(h == null) {
                    h = cur = list1;
                    list1 = list1.next;
                    h.next = null;
                } else {
                    cur.next = list1;
                    list1 = list1.next;
                    cur = cur.next;
                    cur.next = null;
                }
            } else {
                if(h == null) {
                    h = cur = list2;
                    list2 = list2.next;
                    h.next = null;
                } else {
                    cur.next = list2;
                    list2 = list2.next;
                    cur = cur.next;
                    cur.next = null;
                }
            }
        }
        if (list1 !=null) {
            cur.next = list1;
        }
        if(list2 != null) {
            cur.next = list2;
        }
        return h;
    }
}
// @lc code=end