Revise paste 059aa6f750855

#include <iostream>
#include <vector>
using namespace std;
typedef long long ll;
const int maxn = 1000005;

int segment_tree[3 * maxn];

int merge_nodes(int A, int B) {
    return A + B;
}
void build_tree(int L, int R, int node) {
    if(L == R) {
        segment_tree[node] = 0;
        return;
    }
    
    int middle = (L + R) / 2;
    build_tree(L, middle, 2 * node);
    build_tree(middle + 1, R, 2 * node + 1);
    segment_tree[node] = merge_nodes(segment_tree[2 * node], segment_tree[2 * node + 1]);
}

// L R i L R j L R
int query(int i, int j, int L, int R, int node) {
    if(i <= L and R <= j) {
        return segment_tree[node];
    }
    if(R < i or j < L) {
        return 0;
    }
    
    int middle = (L + R) / 2;
    return merge_nodes(query(i, j, L, middle, 2 * node), query(i, j, middle + 1, R, 2 * node + 1));
    
}
void update(int idx, int new_value, int L, int R, int node) {
    if(L == R) {
        segment_tree[node] = new_value;
        return;
    }
    int middle = (L + R) / 2;
    
    if(idx <= middle) {
        update(idx, new_value, L, middle, 2 * node);
    }
    else {
        update(idx, new_value, middle + 1, R, 2 * node + 1);
    }
    
    segment_tree[node] = merge_nodes(segment_tree[2 * node], segment_tree[2 * node + 1]);
    
}
int main() {
    ios_base::sync_with_stdio(false);
    int n;
    cin >> n;
    
    vector<int> v(n);
    for(int i = 0; i < n; i++) {
        cin >> v[i];
    }
    
    build_tree(0, n, 1);
    ll res = 0;
    
    for(int i = n - 1; i >= 0; i--) {
        update(v[i], 1, 0, n, 1);
        res += query(0, v[i] - 1, 0, n, 1);
    }
    cout << res << endl;

return 0;
}