Add Strings

Google Interview Question

Poll of the Day

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Video Solution (Short)

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Video Solution (Detailed)

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Problem Overview

Difficulty: Easy
LeetCode Pattern: Two Pointers

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

Note: You must not convert the inputs to integers directly.

Input: 
· num1 = "456"
· num2 = "77"

Output: "533"

Input: 
· num1 = "11"
· num2 = "123"

Output: "134"

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Step 1: Clarify Requirements

• Can inputs contain negative numbers?

  • No.

• Can inputs contain leading zeros?

  • No.

• Can inputs be of different lengths?

  • Yes.

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Step 2: Discuss Approaches

Approach 1: Convert to int

• Logic:

  • Convert strings to integers

  • Add normally

  • Convert back to string

• Big O:

  • Time Complexity: O(s)

  • Space Complexity: O(s)

Approach 2: Class Addition (Optimal)

• Logic:

  • Start from ends of both strings

  • Add digit by digit with carry

  • Build result string backward

  • Reverse at the end

• Big O:

  • Time Complexity: O(s)

  • Space Complexity: O(s)

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Step 3: Write Code (Optimal)

Python

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Java

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C++

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Step 4: Answer Follow-Ups

• What if input contains non-digit chars?

  • Add input validation.

  • Throw an error or return -1.

• What if inputs have leading zeros?

  • Our algorithm still works.

• What if you had to subtract strings?

  • We will use a similar approach.

  • Just handle borrow carefully.

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