### 问题描述

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.

### 代码

class Solution {
public:

int l1 = 0;
int l2 = 0;

while(p) {
l1++;
p=p->next;
}
while(q) {
l2++;
q=q->next;
}
int gap = l1 - l2;