【唉

primality test

public static boolean isPrime(int N) {
    if(N < 2) return false;
    for(int i = 2; i * i < N; i++)
        if(N % i == 0) return false;
    return true;
}

square root (Newton's method)

public static double sqrt(double c){
    if (c < 0) return Double.NaN;
    double err = le-15;
    double t = c;
    while (Math.abs(t - c/t) > err * t)
        t = (t + c/t) / 2.0;
    return t;
}

Harmonic number

public static double H(int N){
    double sum = 0.0;
    for(int i=1; i < N; i++)
        sum += 1.0/i;
    return sum;
}

Binary Research

public static int rank (int key; int[] a) {
    return rank(key, a, 0, a.length - 1);
}
public static int rank (int key; int[] a; int lo; int hi) {
    if (lo > hi) return -1;
    int mid = lo + (lo + hi)/2;
    if (key < a[mid]) return rank(key, a, lo, mid - 1);
    else if (key > a[mid]) return rank(key, a, mid + 1, hi);
    else                          return mid;
}