2021-06-03

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        //把所有数字压入栈中
        Stack<Integer> stack1=new Stack();
        Stack<Integer> stack2=new Stack();
        while(l1!=null){
            stack1.push(l1.val);
            l1=l1.next;
        }
        while(l2!=null){
            stack2.push(l2.val);
            l2=l2.next;
        }
        //取出来 做+  
        //默认进位为0
        int carry=0;
        ListNode res=new ListNode(-1);//res.next就是结果
      
        while(!stack1.isEmpty()||!stack2.isEmpty()||carry==1){
              int sum=0;
              sum+=stack1.isEmpty()?0:stack1.pop();
              sum+=stack2.isEmpty()?0:stack2.pop();
              sum+=carry;
              ListNode node=new ListNode(sum%10);
              node.next=res.next;//头插法插入
              res.next=node;//头插法插入
              carry=sum/10;
        }
        return  res.next;
    }
}