leetcode: Merge Two Sorted Lists

两个有序链表的插入，类似于归并。两个指针代表两个有序链表，逐个比较将结果插入返回的链表即可

/**
 * 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;
        }
        if( l2 == null )
        {
            return l1;
        }
        ListNode st = new ListNode(0);
        ListNode res = st;
        while( l1 != null && l2 != null )
        {
            if( l1.val < l2.val )
            {
                st.val = l1.val;
                l1 = l1.next;
            }
            else
            {
                st.val = l2.val;
                l2 = l2.next;
            }
            if(l1 != null || l2 != null )
            {
                st.next = new ListNode(0);
                st = st.next;
            }
        }
        if( l2 != null )
        {
            st.val = l2.val;
            st.next = l2.next;
        }
        if( l1 != null )
        {
            st.val = l1.val;
            st.next = l1.next;
        }
        return res;
    }
}