LeetCode 415. 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"


  • 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?


Convert to integer and do the summing. Then convert back to string.

Python Solution

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        return str(int(num1) + int(num2))
  • Time Complexity: O(N).
  • Space Complexity: O(1).

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