[leetcode] [leetcode]Intersection of Two Linked Lists

https://leetcode.com/problems/intersection-of-two-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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
		if (headA == null || headB == null) {
			return null;
		}
		int off = off(headA, headB);
		if (off >= 0) {
			while (off-- > 0) {
				headA = headA.next;
			}
		} else {
			while (off++ < 0) {
				headB = headB.next;
			}
		}
		while (headA != headB) {
			headA = headA.next;
			headB = headB.next;
		}
		return headA;
	}

	// return off > 0, the first is longer, else the second is longer
	private int off(ListNode a, ListNode b) {
		while (a != null && b != null) {
			a = a.next;
			b = b.next;
		}
		int k = 0;
		if (a != null) {
			while (a != null) {
				++k;
				a = a.next;
			}
			return k;
		} else {
			while (b != null) {
				++k;
				b = b.next;
			}
			return -k;
		}
	}
}