415. Add Strings

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

  public String addStrings(String num1, String num2) {  
     int m = num1.length(), n = num2.length();  
     if(m == 0) return num2;  
     if(n == 0) return num1;  
     char[] ans = new char[Math.max(m,n)+1];  
     int carry = 0;  
     int i = m-1, j = n-1, k = Math.max(m,n);  
     for(; i >=0 || j>=0 || carry!=0;i--,j--){  
       int sum = (i>=0? num1.charAt(i)- '0':0) + (j>=0? num2.charAt(j)- '0' :0) + carry;  
       ans[k--] =(char)(sum%10+'0');  
       carry = sum/10;  
     }  
     String s = "";  
     while(++k<Math.max(m,n)+1){  
       s+=ans[k];  
     }  
     return s;  
   }