两个链表的第一个公共结点 --剑指offer

import java.util.*;
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        Stack<ListNode> s1=new Stack<>();
        Stack<ListNode> s2=new Stack<>();
        while (pHead1 != null){
            s1.push(pHead1);
            pHead1=pHead1.next;
        }
        while (pHead2 != null){
            s2.push(pHead2);
            pHead2=pHead2.next;
        }
        ListNode result=null;
        while (!s1.isEmpty() && !s2.isEmpty() && s1.peek() ==s2.peek()){
            s1.pop();
            result =s2.pop();
        }
        return  result;
    }
}

import java.util.*;
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        if(pHead1 == null || pHead2 == null){
            return  null;
        }
        ListNode l1=pHead1,l2=pHead2;
        while(pHead1 != pHead2){
            if(pHead1 != null) pHead1 = pHead1.next;
            else pHead1=l1;
            if(pHead2 != null) pHead2 = pHead2.next;
            else  pHead2=l2;
        }
        return pHead1;
    }
}