Added pbds code

Data Structures/Ordered-Statistic-Tree(pbds).cpp

@@ -0,0 +1,64 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+using namespace __gnu_pbds;
+#define bolt                 \
+    ios::sync_with_stdio(0); \
+    cin.tie(0);              \
+    cout.tie(0);
+#define endl "\n"
+
+#define int long long
+const int N = 2e5 + 5;
+
+#define T pair<int, int>
+typedef tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update> oset;
+#define ll long long
+
+// find1_by_order(k)  returns iterator to kth element starting from 0;
+// order_of_key(k) returns count of elements strictly smaller than k;
+
+/*1. Number of elements strictly greater than a[i]
+int cnt = sz(curr) - (int)curr.order_of_key({a[i]+1,0});
+
+2. Number of elements strictly less than a[i]
+int cnt = (int)curr.order_of_key({a[i],i});
+
+3. Use insert like this : curr.insert({a[id], id});
+
+4. Finding the k -th smallest element in the set using * because
+    find_by_order returns an iterator
+    auto k-th_smallest_value =  *(o_set.find_by_order(k - 1))
+    It is (k - 1) since it uses zero indexes
+
+5. Erasing : curr.erase({a[i], i});*/
+
+/*
+Example : 
+Problem : https://codeforces.com/contest/1042/problem/D
+Solution :
+*/
+
+ll a[200005], n, k, prefix[200005], ans;
+
+signed main()
+{
+#ifdef ONLINE_JUDGE
+    bolt;
+#endif
+    cin >> n >> k;
+    oset s;
+    s.insert({0, 0});
+    for (int i = 1; i <= n; ++i)
+    {
+        cin >> a[i];
+        prefix[i] = prefix[i - 1] + a[i];
+        int now = s.order_of_key({prefix[i] - k, 1e9});
+        ans += s.size() - now;
+        s.insert({prefix[i], i});
+    }
+    cout << ans;
+    return 0;
+}

README.md

@@ -23,6 +23,7 @@ This Repo is For those who want to contribute in an open Source Repo for Hacktob
 ├── Data Structures
 │   ├── DSU.cpp
 │   ├── Mo's-algorithm.cpp
+│   ├── Ordered-Statistic-Tree(pbds).cpp
 ├── Divide and conquer
 │   ├── maximum-subarray-sum-O(nlogn).cpp
 │   ├── merge-sort-O(nlogn).cpp