算法(二分查找)

public static int binarySearch(int x) {
    int l = 0, r = x, index = -1;
    while (l <= r) {
        int mid = l + (r-l)/2;
        if ((long)mid * mid <= x) {
            index = mid;
            l = mid + 1;
        } else {
            r = mid - 1;
        }
    }
    return index;
}

// 牛顿迭代 尚需要了解(时间复杂度更低)