题解 | #合并两个排序的链表# 一次遍历O(n)

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param pHead1 ListNode类 
     * @param pHead2 ListNode类 
     * @return ListNode类
     */
    public ListNode Merge (ListNode pHead1, ListNode pHead2) {
        if(pHead1 == null) return pHead2;
        else if(pHead2 == null) return pHead1;
        // write code here
        ListNode pre = new ListNode(-1),cur = new ListNode(-1);
        pre = cur;
        while(pHead1 != null || pHead2 != null){
            if(pHead1 == null){
                cur.next = pHead2;
                break;
            }else if(pHead2 == null){
                cur.next = pHead1;
                break;
            }
            if(pHead1.val <= pHead2.val){
                cur.next = pHead1;
                pHead1 = pHead1.next;
                cur = cur.next;
            }else {
                cur.next = pHead2;
                pHead2 = pHead2.next;
                cur = cur.next;
            }
        }
        return pre.next;
    }
}