Intersection of Two Linked Lists

Problem

Two singly linked lists eventually merge. Return the node where they first intersect (by reference), or null.

Constraints

• 1 ≤ each list length ≤ 10⁴

Approach — Two Pointers

This is a Two Pointers problem. The idea: walk two indices through the data together (or toward each other), turning an O(n²) pair search into one linear pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: O(n) time, O(1) extra space.

Solution code

Python

class Solution:
    def getIntersectionNode(self, headA, headB):
        a, b = headA, headB
        while a is not b:
            a = a.next if a else headB
            b = b.next if b else headA
        return a

Java

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

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.