160. Intersection of Two Linked Lists

// Approach: Two pointers switch to the other list when reaching null; both
// traverse the same total distance and meet at the intersection.
// Time: O(m+n) Space: O(1)

public class ListNode
{
    public int val;
    public ListNode next;
    public ListNode(int x) { val = x; }
}

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)
        {
            a = a == null ? headB : a.next;
            b = b == null ? headA : b.next;
        }
        return a;
    }
}