public class Solution {
 /**
 * @param num1 a non-negative integers
 * @param num2 a non-negative integers
 * @return return sum of num1 and num2
 */
 public String addStrings(String num1, String num2) {
 // Write your code here
 	if (num2.length() > num1.length()) {
 		return addStrings(num2, num1);
 	}
 	int p1 = num1.length()-1, p2 = num2.length()-1;
 	int[] sum = new int[p1+1];
 	int carry = 0, a, b;
 	for (; p2 >= 0; --p1, --p2) {
 		a = num1.charAt(p1) - '0';
 		b = num2.charAt(p2) - '0';
 		sum[p1] = a + b + carry;
 		if (sum[p1] >= 10) {
 			carry = sum[p1] / 10;
 			sum[p1] %= 10;
 		} else {
 			carry = 0;
 		}
 	}
 	for (; p1 >= 0; --p1) {
 		a = num1.charAt(p1) - '0';
 		sum[p1] = a + carry;
 		if (sum[p1] >= 10) {
 			carry = sum[p1] / 10;
 			sum[p1] %= 10;
 		} else {
 			carry = 0;
 		}
 	}
 	StringBuilder sb = new StringBuilder();
 	if (carry > 0) {
 		sb.append(carry);
 	}
 	for (int i=0; i<sum.length; ++i) {
 		sb.append(sum[i]);
 	}
 	return sb.toString();
 }
}