public boolean isPerfectSquare(int num) {
    long low = 0, high = num;
    while(low <= high) {
        long mid = low + (high - low)/2;
        //int product = mid * mid;
        // if(mid != 0 && product/mid != mid) {
        //     high = mid - 1; // integer overflow;
        //     continue;
        // }
        if(mid * mid == num) {
            return true;
        }
        if(mid * mid > num) {
            high = mid - 1;
        } else {
            low = mid + 1;
        }
    }
    return false;
}