160. Intersection of Two Linked Lists

题目160. 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.

思路:
- 求得两个链表长度lenA,lenB. 这里假设lenA>lenB,
- 链表A前进(lenB-lenA)个节点,这样和链表B对齐
- 链表A和链表B同时向前每次移动一个节点,进行对比,如果相等返回这个节点,如果两者都到链表尾部不
相等,则返回null

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = getListLength(headA);
        int lenB = getListLength(headB);
        
        while(lenA > lenB){
            headA = headA.next;
            lenA--;
        }
        
        while(lenA < lenB){
            headB = headB.next;
            lenB--;
        }
        
        while(headA != headB){
            headA = headA.next;
            headB = headB.next;
        }
        return headA;
    }
    
    private int getListLength(ListNode head){
        int len = 0;
        ListNode node = head;
        while(node != null){
            len++;
            node = node.next;
        }
        return len;
    }