# Intersection of Linked Lists

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.

```java
public class Solution {
    public int length(ListNode node) {
        int len = 0;
        while (node != null) {
            len++;
            node = node.next;
        }
        return len;
    }

    public ListNode getIntersectionNode(ListNode a, ListNode b) {
        int len1 = length(a);
        int len2 = length(b);
        if (len1 > len2) {
            int diff = len1 - len2;
            while (diff > 0) {
                a = a.next;
                diff--;
            }
        } else if (len1 < len2) {
            int diff = len2 - len1;
            while (diff > 0) {
                b = b.next;
                diff--;
            }
        }
        while (a != null && b != null) {
            if (a == b)
                return a;
            a = a.next;
            b = b.next;
        }
        return null;
    }
}
```
