Math - Add Strings

Given two non-negative integers num1 and num2 represented as strings, return the sum of these two numbers, also as a string. You must not use any built-in BigInteger library.

Input: "11" + "123" → Output: "134"

Input: "456" + "77" → Output: "533"

Simulate grade-school addition from right to left, carrying over.

Two pointers at end of both strings, carry=0.
While i>=0 or j>=0 or carry>0: sum = carry + (num1[i] if valid) + (num2[j] if valid).
Append sum%10, carry = sum/10.
Reverse result.

class Solution { public String addStrings(String num1, String num2) { StringBuilder sb = new StringBuilder(); int i = num1.length()-1, j = num2.length()-1, carry = 0; while (i >= 0 || j >= 0 || carry > 0) { int sum = carry; if (i >= 0) sum += num1.charAt(i--) - '0'; if (j >= 0) sum += num2.charAt(j--) - '0'; sb.append(sum % 10); carry = sum / 10; } return sb.reverse().toString(); } }

Time Complexity: O(max(M,N))
Space Complexity: O(max(M,N))