Intersection of Two Linked Lists leetcode

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.

首先找到两条链表的长度, 然后截取相同的长度, 从两个开端一起走 碰到一起的话返回碰到时候的node, 没有相遇的话返回null.

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lena = getlen(headA);
        int lenb = getlen(headB);
        ListNode nodea = headA;
        ListNode nodeb = headB;
        if (lena > lenb) {
            for (int i = 0; i < lena - lenb; i++) {
                nodea = nodea.next;
            }
        } else {
            for (int i = 0; i < lenb - lena; i++) {
                nodeb = nodeb.next;
            }
        }
        while (nodea != null && nodeb!= null) {
            if (nodea == nodeb) {
                return nodea;
            }
            nodea = nodea.next;
            nodeb = nodeb.next;
        }
        return null;
    }
    public int getlen(ListNode head) {
        int count = 0;
        while (head != null) {
            head = head.next;
            count++;
        }
        return count;
    } 
}