Java [Leetcode 160]Intersection of Two Linked Lists

题目描述：

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

解题思路 ：

主要是考虑两条链路不一样长的问题，那么在其中一条链路到底的时候立马换回到另一条链路上面，另一条链路到底的时候也换回到这一条链路上，这样保证了后面能够同时到达相交点。

代码如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {

    	if(headA == null || headB == null)
        	return null;

        ListNode a = headA;
        ListNode b = headB;

        while(a != b){
        	if(a == null)
        		a = headB;
        	else
        		a = a.next;
        	if(b == null)
        		b = headA;
        	else
        		b = b.next;
        }

        return a;
    }
}