LeetCode 415. Add Strings

Description

https://leetcode.com/problems/add-strings/

Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.

Example 1:

Input: num1 = "11", num2 = "123"
Output: "134"

Example 2:

Input: num1 = "456", num2 = "77"
Output: "533"

Example 3:

Input: num1 = "0", num2 = "0"
Output: "0"

Constraints:

  • 1 <= num1.length, num2.length <= 104
  • num1 and num2 consist of only digits.
  • num1 and num2 don’t have any leading zeros except for the zero itself.

Follow up: Could you solve it without using any built-in BigInteger library or converting the inputs to integer directly?

Explanation

Convert strings to integers and sum up to get the sum integer. Then convert sum integer to a string.

Python Solution

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        
        
        num1_int = 0
        
        for i in range(0, len(num1)):            
            num1_int = 10 * num1_int + int(num1[i])
        
                        
        num2_int = 0
        
        for i in range(0, len(num2)):            
            num2_int = 10 * num2_int + int(num2[i])
        
        
        sum_int = num1_int + num2_int
        
        if sum_int == 0:
            return "0"
        
        sum_str = ""
        while sum_int > 0:
            
            sum_str = str(sum_int % 10) + sum_str
            
            sum_int //= 10
            
            
        return sum_str
            
  • Time Complexity: O(N).
  • Space Complexity: O(1).

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