LeetCode 445. Add Two Numbers II (两数相加 II)

题目标签：Linked List

　　题目给了我们两个 数字的linked list，让我们把它们相加，返回一个新的linked list。

　　因为题目要求不能 reverse，可以把 两个list 的数字都 存入 两个stack。利用了stack的特性，就可以从后面把数字相加，具体看code。

Java Solution:

Runtime:  6 ms, faster than 51.06% 

Memory Usage: 45.4 MB, less than 64.71%

完成日期：07/08/2019

关键点：stack

/**
 * 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> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        
        // store two numbers into stacks
        while(l1 != null) {
            s1.push(l1.val);
            l1 = l1.next;
        }
        
        while(l2 != null) {
            s2.push(l2.val);
            l2 = l2.next;
        }
        
        // adding two numbers from stacks
        int sum = 0;
        ListNode list = new ListNode(0);
        
        while(!s1.empty() || !s2.empty()) {
            if(!s1.empty())
                sum += s1.pop();
            if(!s2.empty())
                sum += s2.pop();
            
            list.val = sum % 10; // get the value
            ListNode head = new ListNode(sum / 10); // get the carry
            head.next = list;
            list = head;
            
            sum /= 10;
        }
        
        
        // need to check if the head is 0 or not
        return list.val == 0 ? list.next : list;
    }
}

参考资料：LeetCode Discuss

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题目来源：https://leetcode.com/