class Solution {
    public String multiply(String num1, String num2) {
        int n = num1.length(), m = num2.length();
        int[] a = new int[n];
        int[] b = new int[m];
        for(int i = n - 1; i >= 0; i -- ) a[n - 1 - i] = num1.charAt(i) - '0';
        for(int i = m - 1; i >= 0; i -- ) b[m - 1 - i] = num2.charAt(i) - '0';
        int[] c = new int[n + m];
        for(int i = 0; i < n; i ++ )
            for(int j = 0; j < m; j ++ )
                c[i + j] += a[i] * b[j];

        for(int i = 0, t = 0; i < n + m; i ++ ){
            t += c[i];
            c[i] = t % 10;
            t /= 10;
        }

        int k = n + m - 1;
        while(k > 0 && c[k] == 0) k -- ;
        StringBuilder res = new StringBuilder();
        while(k >= 0) res.append(c[k -- ]);

        return res.toString();
    }
}