FenwickTree

Computes partial sums a[0] + a[1] + ... + a[pos - 1], and updates single elements a[i], taking the difference between the old and new value.

Time: Both operations are O(log N). 22 lines Stress-tested

content/data-structures/FenwickTree.h — Lukas Polacek, source: folklore/TopCoder

struct FT {
	vector<ll> s;
	FT(int n) : s(n) {}
	void update(int pos, ll dif) { // a[pos] += dif
		for (; pos < sz(s); pos |= pos + 1) s[pos] += dif;
	}
	ll query(int pos) { // sum of values in [0, pos)
		ll res = 0;
		for (; pos > 0; pos &= pos - 1) res += s[pos-1];
		return res;
	}
	int lower_bound(ll sum) {// min pos st sum of [0, pos] >= sum
		// Returns n if no sum is >= sum, or -1 if empty sum is.
		if (sum <= 0) return -1;
		int pos = 0;
		for (int pw = 1 << 25; pw; pw >>= 1) {
			if (pos + pw <= sz(s) && s[pos + pw-1] < sum)
				pos += pw, sum -= s[pos-1];
		}
		return pos;
	}
};