public static void main(String[] args) {

System.out.println(

new leetcode415().addStrings("9","52")

);

}

public String addStrings(String num1, String num2) {

if (num1.length()<num2.length()){

return addStrings(num2,num1);

}

int m = num1.length(), n = num2.length();

int[] pos = new int[m+1];

for(int i=m-1,j=n-1; i >= 0; i--) {

int add=0;

if (j>=0){

add = (num1.charAt(i) - '0') + (num2.charAt(j) - '0');

j--;

}else

add = (num1.charAt(i) - '0') +pos[i];

int p1 = i , p2 = i + 1;

int sum = add + pos[p2];

pos[p1] += sum / 10;

pos[p2] = (sum) % 10;

}

StringBuilder sb = new StringBuilder();

for(int p : pos) if(!(sb.length() == 0 && p == 0)) sb.append(p);

return sb.length() == 0 ? "0" : sb.toString();

}