Add strings

Math String Simulation

52.1% acceptance

Jan 13, 2026

5435

829

Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string. You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn add_strings(num1: String, num2: String) -> String {
    let mut result = Vec::new();
    let mut carry = 0;
    let mut i = num1.len() as i32 - 1;
    let mut j = num2.len() as i32 - 1;
    let num1: Vec<u8> = num1.bytes().collect();
    let num2: Vec<u8> = num2.bytes().collect();
    
    while i >= 0 || j >= 0 || carry > 0 {
      let digit1 = if i >= 0 { (num1[i as usize] - b'0') as i32 } else { 0 };
      let digit2 = if j >= 0 { (num2[j as usize] - b'0') as i32 } else { 0 };
      
      let sum = digit1 + digit2 + carry;
      result.push((sum % 10) as u8 + b'0');
      carry = sum / 10;
      
      i -= 1;
      j -= 1;
    }
    
    result.reverse();
    String::from_utf8(result).unwrap()
  }
}