【剑指offer】链表第一个公共子结点

*思路： 先求得两个链表的长度，然后得到长度差diff，再先遍历长链表diff步后，再同时遍历两个链表并比较对象指针。

 /*
 public class ListNode {
     int val;
     ListNode next = null;

     ListNode(int val) {
         this.val = val;
     }
 }*/
 public class Solution {
     public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
       if(pHead1==null||pHead2==null)
         {return null;}
       ListNode temp1 = pHead1;
       ListNode temp2 = pHead2;
       int length1=0;
       int length2=0;
       while(temp1!=null){
           length1++;
           temp1 = temp1.next;
       }
       while(temp2!=null){
           length2++;
           temp2 = temp2.next;
       }
       int diff= Math.abs(length1-length2);
       if(length1>=length2){
           for(int i=0; i<diff; i++){
               pHead1 = pHead1.next;
           }
       }else{
           for(int i=0; i<diff; i++){
               pHead2 = pHead2.next;
           }
       }
       while(pHead1!=pHead2){
           pHead1 = pHead1.next;
           pHead2 = pHead2.next;
       }

       return pHead1;

     }
 }