public class Solution {

public String addStrings(String num1, String num2) {

if ("".equals(num1))

return num2;

if ("".equals(num2))

return num1;

int loopTimes = Math.max(num1.length(), num2.length()) + 1;

int resultLength = loopTimes;

char[] result = new char[loopTimes];

int carry = 0;

int num1Index = num1.length() - 1;

int num2Index = num2.length() - 1;

int operandA = num1.charAt(num1Index--) - '0';

int operandB = num2.charAt(num2Index--) - '0';

while (loopTimes-- > 0) {

int bitSum = (operandA + operandB + carry);

result[loopTimes] = (char)(bitSum % 10 + '0');

carry = bitSum / 10;

if (num1Index < 0) {

operandA = 0;

} else {

operandA = num1.charAt(num1Index--) - '0';

}

if (num2Index < 0) {

operandB = 0;

} else {

operandB = num2.charAt(num2Index--) - '0';

}

}

int idx = 0;

while (idx < resultLength - 1 && result[idx] == '0')

idx++;

StringBuilder sb = new StringBuilder();

while (idx < resultLength) {

sb.append(result[idx++]);

}

return sb.toString();

}

}