Big Number Multiply

    public String multi(String a, String b){
        ////反转字符串
        char[] ar = reverse(a).toCharArray(), br = reverse(b).toCharArray();

        int res[] = new int[1000];
        int max = ar.length + br.length-1;//m位数乘n位数，结果至少是m+n-1位

        for (int i = 0; i <max; i++) {
            for (int j = 0;  i < ar.length && j < br.length; j++) {
                res[i + j] += (ar[i] - 48)*(br[j] - 48);
            }

            if (res[i] >= 10) {
                res[i+1] += res[i]/10;
                res[i] %= 10;
                max= Math.max(max, i+2);//结果最多是m+n位，+2是因为i是从0开始的下标，m是从1开始的位数
            }
        }

        
        StringBuilder ans = new StringBuilder();
        for (int i = 0;i<max; i++) {
            ans.append((char)(res[i]+48));
        }

        //反转结果
        return ans.reverse().toString();
    }

    public String reverse(String s){
        return new StringBuilder(s).reverse().toString();
    }