Problem

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

Note:

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

Solution: Brute Force

Time complexity: O(n)

Space complexity: O(n)

C++

// Author: Huahua

// Running time: 13 ms

class Solution {

public:

string addStrings(string num1, string num2) {

if (num1.length() < num2.length()) swap(num1, num2);

num1 = "0" + num1;

int l1 = num1.length();

int l2 = num2.length();

for (int i = 0; i < l1 - 1; ++i) {

if (i < l2) num1[l1 - i - 1] += (num2[l2 - i - 1] - '0');

if (num1[l1 - i - 1] > '9') {

num1[l1 - i - 1] -= 10;

++num1[l1 - i - 2];

}

return (num1[0] != '0') ? num1 : num1.substr(1);

}

};

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