<Math> 258 43

258. Add Digits

 

 

 

class Solution {
    public int addDigits(int num) {
        if(num == 0)
            return 0;
        if(num % 9 == 0){
            return 9;
        }else{
            return num % 9;
        }
    }
}

43. Multiply Strings

Primary multiplication, the calculated maximum numbers have i + j bits.

Finally, to ensure that the first bit is not equal to 0 is added to the tail of StringBuilder.

class Solution {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] pos = new int[m + n];
        
        for(int j = n - 1; j >= 0; j--){
            for(int i = m - 1; i >= 0; i--){
                int product = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                int p1 = i + j, p2 = i + j + 1;
                int sum = product + 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();

} }