二叉索引树（树状数组）

//inline int lowbit(int x) { return x&(x^(x-1)); }
inline int lowbit(int x) { return x&-x; }

struct FenwickTree {
int n;
vector<int> C;

void resize(int n) { this->n = n; C.resize(n); }
void clear() { fill(C.begin(), C.end(), 0); }

// 计算A[1]+A[2]+...+A[x] (x<=n)
int sum(int x) {
int ret = 0;
while(x > 0) {
ret += C[x]; x -= lowbit(x);
}
return ret;
}

// A[x] += d (1<=x<=n)
void add(int x, int d) {
while(x <= n) {
C[x] += d; x += lowbit(x);
}
}
};