• Time:O(n)
• Space:O(n)

## C++

``````class Solution {
public:
string addStrings(string num1, string num2) {
string ans;
int carry = 0;
int i = num1.length() - 1;
int j = num2.length() - 1;

while (i >= 0 || j >= 0 || carry) {
if (i >= 0)
carry += num1[i--] - '0';
if (j >= 0)
carry += num2[j--] - '0';
ans += carry % 10 + '0';
carry /= 10;
}

reverse(begin(ans), end(ans));
return ans;
}
};
``````

## JAVA

``````class Solution {
public String addStrings(String num1, String num2) {
StringBuilder sb = new StringBuilder();
int carry = 0;
int i = num1.length() - 1;
int j = num2.length() - 1;

while (i >= 0 || j >= 0 || carry > 0) {
if (i >= 0)
carry += num1.charAt(i--) - '0';
if (j >= 0)
carry += num2.charAt(j--) - '0';
sb.append(carry % 10);
carry /= 10;
}

return sb.reverse().toString();
}
}
``````

## Python

``````class Solution:
def addStrings(self, num1: str, num2: str) -> str:
ans = []
carry = 0
i = len(num1) - 1
j = len(num2) - 1

while i >= 0 or j >= 0 or carry:
if i >= 0:
carry += int(num1[i])
if j >= 0:
carry += int(num2[j])
ans.append(str(carry % 10))
carry //= 10
i -= 1
j -= 1

return ''.join(reversed(ans))
``````