题目：合并两个有序链表

算法：就地合并（不使用额外内存空间）

/**
 * 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) {
	        // emtpy list
	        if (null==l1 && null==l2) {
	        	return null;
	        } else if (null == l1) {
	        	return l2;
	        } else if (null == l2) {
	        	return l1;
	        }
	    	
	    	ListNode pa = l1;
	        ListNode pb = l2;
	        ListNode pc = null;
	        ListNode head = null;
	        
	        if (pa.val <= pb.val) {
	        	pc = pa;
	        	pa = pa.next;
	        } else {
	        	pc = pb;
	        	pb = pb.next;
	        }
	        head = pc;
	        
	        while (null!=pa && null!=pb) {
	        	if (pa.val <= pb.val) {
	        		pc.next = pa;
	        		pa = pa.next;
	        	} else {
	        		pc.next = pb;
	        		pb = pb.next;
	        	}
	        	
	        	pc = pc.next;
	        }
	        if (null == pa) {
	        	pc.next = pb;
	        } else if (null == pb) {
	        	pc.next = pa;
	        }
	        
	        return head;
	    }
}