445. 两数相加 II

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/add-two-numbers-ii
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最高位变成了头结点，这样就需要从后向前遍历。借用栈的性质

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        Stack<ListNode> stack1 = putStack(l1);
        Stack<ListNode> stack2 = putStack(l2);
        ListNode head = null;
        int carry = 0;
        while(!stack1.isEmpty() || !stack2.isEmpty()) {
            int num1 = stack1.isEmpty() ? 0 : stack1.pop().val;
            int num2 = stack2.isEmpty() ? 0 : stack2.pop().val;

            int sum = num1 + num2 + carry;
            ListNode cur = new ListNode(sum %10);
            cur.next = head;
            head = cur;
            carry = sum / 10;
        }

        if(carry != 0) {
            ListNode cur = new ListNode(carry);
            cur.next = head;
            head = cur;
        }
        return head;

    }

    
    // 由于最高位位于开始，那么就要用到stack了

    public Stack<ListNode> putStack(ListNode list) {

        Stack<ListNode> stack = new Stack<>();
        while(list != null) {
            stack.push(list);
            list = list.next;
        }
        return stack;

    }