leet code solver

bigNumberMultiplication.java

public class Solution {
    public String multiply(String num1, String num2) {
        return toString(mult(toIntArray(num1), toIntArray(num2)));
    }
    
    private int[] toIntArray(String s) {
        int[] result = new int[s.length()];
        for (int i=0; i<result.length; ++i) {
            result[i] = Character.digit(s.charAt(s.length()-1-i), 10);
        }
        return result;
    }
    
    private String toString(int[] arr) {
        StringBuilder sb = new StringBuilder(arr.length);
        int i=arr.length-1;
        while (arr[i] == 0 && i>0) --i;
        for (; i>=0; --i) {
            sb.append(arr[i]);
        }
        return sb.toString();
    }
    
    private int[] mult(int[] a1, int[] a2) {
        int[] result = new int[a1.length+a2.length];
        for (int i=0; i<a1.length; ++i) {
            int m = a1[i];
            for (int j=i; j<a2.length+i;++j) {
                result[j] += a2[j-i]*m;
            }
        }
        for (int i=0; i<result.length-1; ++i) {
            result[i+1]+=result[i] / 10;
            result[i] = result[i] % 10;
        }
        return result;
    }
}