相交链表

/**
 * 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;
        if(headA==null && headB==null) return null;
        int lenA=0; //链表A长度
        int lenB=0; //链表B长度
        ListNode a=headA;
        ListNode b=headB;
        //求出headA的长度
        while(a!=null){
            lenA++;
            a=a.next;
        }
        //求出headB的长度
        while(b!=null){
            lenB++;
            b=b.next;
        }
        a=headA;
        b=headB;
        //判断哪个链表比较长
        if(lenA>lenB){
            int diff=lenA-lenB;
            //先让headA走diff步
            for(int i=1;i<=diff;i++){
                a=a.next;
            }
        }else{
           int diff=lenB-lenA;
             //先让headB走diff步
            for(int i=1;i<=diff;i++){
                b=b.next;
            }
        }
        while (a != null) {
            if (a == b) return a; 
            else {
                a = a.next;
                b = b.next;
            }
        }
        return null;
    }
}