Code. Implementing Fenwick trees with bit tricks in Java [blog/fenwick/java_fenwick]

class FenwickTree {
    private long[] a;
    public FenwickTree(int n) { a = new long[n+1]; }
    public long prefix(int i) {
        long s = 0;
        for (; i > 0; i -= LSB(i)) s += a[i]; return s;
    }
    public void update(int i, long delta) {
        for (; i < a.length; i += LSB(i)) a[i] += delta;
    }
    public long range(int i, int j) {
        return prefix(j) - prefix(i-1);
    }
    public long get(int i) { return range(i,i); }
    public void set(int i, long v) { update(i, v - get(i)); }
    private int LSB(int i) { return i & (-i); }
}