【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;
      
    }
}