Intersection of Two Linked Lists

Problem

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.

Solution

Amazing Two Pointers Solution: O(n) time, O(1) space

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null) return null;
        ListNode a = headA, b = headB;
        while (a != b) {
            a = a == null ? headB : a.next;
            b = b == null ? headA : b.next;
        }
        return a; //or b, anyway 
    }
}

Analysis

To prove above solution work, let's take headA = [1,2,3,4,5,6] and headB = [7,8,9,5] as example
Since headA has two nodes more than headB, b will be set to headA earlier
After two more steps, a will be set to headB and b passed two nodes already
Therefore, we guarantee that a and b will meet exactly at the intersection
If there is no intersection, they will be initially both set to null hence we will return null at the end