LC.160. Intersection of Two Linked Lists

https://leetcode.com/problems/intersection-of-two-linked-lists/description/

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.

//time:o(n) space:o(1)

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null) {
            return null;
        }
        int lengthA = getLength(headA);
        int lengthB = getLength(headB);
        ListNode currA = headA;
        ListNode currB = headB;
        if (lengthA > lengthB) {
            while (lengthA != lengthB) {
                currA = currA.next;
                lengthA--;
            }
        } else {
            while (lengthB != lengthA){
                currB = currB.next ;
                lengthB--;
            }
        }
        //now they are the same length
        while (currA!=currB ){
            currA = currA.next ;
            currB = currB.next ;
        }
        //out of while, means currA == currB, including both == null
        return currA ;
    }

    private int getLength(ListNode head) {
        int length = 1;
        while (head != null) {
            head = head.next;
            length++;
        }
        return length;
    }