Binary Search

    public int mySqrt(int x) {
        if (x == 0 || x == 1) return x;
        int start = 0, end = x;

        while (end - start > 1) {
            int mid = start + (end - start) / 2;
            if (mid <= x / mid) { // Avoid int overflow
                start = mid;
            } else {
                end = mid;
            }
        }
        return start;
    }