160. 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.

Analysis:

Compare A's length and B's length. Trim the longer one to the length of the shorter one. Then start going afterward. 

Solution:

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = len(headA);
        int lenB = len(headB);

        if (lenA < lenB) {
            while (lenA != lenB) {
                headB = headB.next;
                lenB--;
            }
        } else {
            while (lenB != lenA) {
                headA = headA.next;
                lenA--;
            }
        }

        while (headA != headB) {
            headA = headA.next;
            headB = headB.next;
        }

        return headA;
    }

    private int len(ListNode node) {
        int len = 0;
        while (node != null) {
            node = node.next;
            len++;
        }
        return len;
    }
}