7.两数相加（数存储在链表里）

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> stack1=new Stack();//栈的创建
        Stack<Integer> stack2=new Stack();
        ListNode sum=new ListNode(0);
        ListNode p1=l1,p2=l2,p3=sum;
        while(l1!=null){
            stack1.push(l1.val);
            l1=l1.next;

        }
        while(l2!=null){
            stack2.push(l2.val);//压栈
            l2=l2.next;
        }
        ListNode head=null;
        int flag=0;
        int value=0;
        while(!stack1.isEmpty() || !stack2.isEmpty() || flag!=0){
            if(!stack1.isEmpty())
                value+=stack1.pop();//出栈
            if(!stack2.isEmpty())
                value+=stack2.pop();
            value+=flag;
            ListNode p=new ListNode(value%10);
            flag=value/10;
            p.next=head; //注意这里相当于是head和p互换了位置，即head总指向新进来的节点
            head=p;
            value=0;

        }
        return head;

    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode sum=new ListNode(0);
        ListNode p1=l1,p2=l2,p3=sum;
        int flag=0;
        while(p1!=null || p2!=null || flag!=0){
            int value=0;
            if(p1!=null){
                value+=p1.val;
                p1=p1.next;
            }
            if(p2!=null){
                value+=p2.val;
                p2=p2.next;
            }
            value+=flag;
            p3.next=new ListNode(value%10);//将p3当做一个指针
            p3=p3.next;
            flag=value/10;
        }
        return sum.next;

    }
}