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.

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

直接上代码：

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = getLength(headA);
        int lenB = getLength(headB);
        ListNode curA = headA;
        ListNode curB = headB;
        int count = lenA > lenB ? lenA - lenB : lenB - lenA;
        if (lenA > lenB) {
            for (int i = 0; i < count; i++) {
                curA = curA.next;
            }
        }else{
            for (int i = 0; i< count; i++) {
                curB = curB.next;
            }
        }
        while (curA != null && curB != null) {
            if (curA.val == curB.val) {
                return curA;
            }
            curA = curA.next;
            curB = curB.next;
        }
        return null;
    }
    
    private int getLength(ListNode head) {
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        dummy.next = head;
        int count = 0;
        while (cur != null) {
            cur = cur.next;
            count++;
        }
        return count;
    }