21.java

/**
 * 21. Merge Two Sorted Lists
 * 
 * You are given the heads of two sorted linked lists list1 and list2.
 * 
 * Merge the two lists in to one sorted list. The list should be made by splicing together
 * nodes of the first two lists.
 * 
 * Return the head of the merged linked list.
 * 
 * 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) {
        ListNode dummy = new ListNode();
        ListNode current = dummy;
        while (list1 != null) {
            if (list2 != null && list2.val < list1.val) {
                current.next = list2;
                list2 = list2.next;
            } else {
                current.next = list1;
                list1 = list1.next;
            }
            current = current.next;
        }
        while (list2 != null) {
            current.next = list2;
            list2 = list2.next;
            current = current.next;
        }
        if (dummy != null) return dummy.next;
        return dummy;
    }
}

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}