leetcode 445. Add Two Numbers II

最新推荐文章于 2024-11-03 07:15:00 发布

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本文介绍了一种解决两数相加问题的链表实现方法。通过递归或使用栈的方式，实现了两个非空链表所表示的非负整数相加，并返回结果链表。递归方法通过将低位值计算出来连接到高位值；栈方法利用栈先进后出特性，直接进行数值计算。

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

使用递归的方法，把后面的值算出来再连到前面

public class Solution {
    int carry=0;
    int len1=0, len2=0;
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode temp = l1;
        while(temp!=null){temp = temp.next; len1++;}
        temp = l2;
        while(temp!=null){temp = temp.next; len2++;}
        ListNode node = helper(l1, l2);
        if(carry==1) {
            ListNode newnode = new ListNode(1);
            newnode.next = node;
            return newnode;
        }
        return node;
    }
    
    private ListNode helper(ListNode l1, ListNode l2){
        if(l1==null && l2==null) return null;
        ListNode node = new ListNode(0);
        if(len1>len2){
            len1--;
            node.next = helper(l1.next, l2);
            int temp = l1.val + carry;
            node.val = temp % 10;
            carry = temp / 10;
        }else if(len1<len2){
            len2--;
            node.next = helper(l1, l2.next);
            int temp = l2.val + carry;
            node.val = temp % 10;
            carry = temp / 10;
        }else{
            node.next = helper(l1.next, l2.next);
            int temp = l1.val + l2.val + carry;
            node.val = temp % 10;
            carry = temp / 10;
        }
        return node;
    }
}

看到别人用Stack来做，感觉更直观一些

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        
        while(l1 != null) {
            s1.push(l1.val);
            l1 = l1.next;
        };
        while(l2 != null) {
            s2.push(l2.val);
            l2 = l2.next;
        }
        
        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;
            ListNode head = new ListNode(sum / 10);
            head.next = list;
            list = head;
            sum /= 10;
        }
        
        return list.val == 0 ? list.next : list;
    }
}