segment tree problemds code example

Example 1: segment tree

const int N = 1e5;  // limit for array size
int n;  // array size
int t[2 * N];

void build() {  // build the tree
  	for (int i = n - 1; i > 0; --i) t[i] = t[i<<1] + t[i<<1|1];
}

void modify(int p, int value) {  // set value at position p
  	for (t[p += n] = value; p > 1; p >>= 1) t[p>>1] = t[p] + t[p^1];
}

int query(int l, int r) {  // sum on interval [l, r)
    int res = 0;
    for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
        if (l&1) res += t[l++];
        if (r&1) res += t[--r];
    }
    return res;
}

int main() {
	std::cin>>n;
    for (int i = 0; i < n; ++i) std::cin>>t[n+i];
    build();
    modify(0, 1);
    std::cout<<query(3, 11)<<'\n';
    return 0;
}

Example 2: Segment tree

void build(int node, int start, int end)
{
    if(start == end)
    {
        // Leaf node will have a single element
        tree[node] = A[start];
    }
    else
    {
        int mid = (start + end) / 2;
        // Recurse on the left child
        build(2*node, start, mid);
        // Recurse on the right child
        build(2*node+1, mid+1, end);
        // Internal node will have the sum of both of its children
        tree[node] = tree[2*node] + tree[2*node+1];
    }
}