原理

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import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param l1 ListNode类 
     * @param l2 ListNode类 
     * @return ListNode类
     */
    public ListNode mergeTwoLists (ListNode l1, ListNode l2) {
        // write code here
        ListNode head = new ListNode(0);
        ListNode tmp = head;
        while(l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                tmp.next = l1;
                l1 = l1.next;
                tmp = tmp.next;
            } else {
                tmp.next = l2;
                l2 = l2.next;
                tmp = tmp.next;
            }
        }
        if (l1 == null) tmp.next = l2;
        else if (l2 == null) tmp.next = l1;
        return head.next; //这里要排除第一个节点，因为他是 0
    }
}