Leetcode002_addTwoNumbers

Description

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order 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.

Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

Explanation: 342 + 465 = 807.

Solution

Reverse two numbers and add them, then reverse them and store in a linkedlist

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        num1 = ''
        num2 = ''
        while l1:
            num1 += str(l1.val)
            l1 = l1.next
        while l2:
            num2 += str(l2.val)
            l2 = l2.next
        add = str(int(num1[::-1])+int(num2[::-1]))[::-1]
        head = ListNode(add[0])
        answer = head
        for i in range(1, len(add)):
            node = ListNode(add[i])
            head.next = node
            head = head.next
        return answer

/**
 * 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 res = null;
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1 == null && l2 == null) return res;
        
        res = new ListNode(0);
        int sum = 0;
        ListNode head = res;
        
        while(l1!=null || l2!=null){
            if (l1 != null){
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null){
                sum += l2.val;
                l2 = l2.next;
            }
            res.next = new ListNode(sum%10);
            res = res.next;
            sum /= 10;
        }
        if (sum > 0) res.next = new ListNode(sum);
        
        return head.next;
    }
}

Note

In python, using

str[::-1]

to reverse a string is a most simple way

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