Add Day-09 Salary Queries solution (C++)

Author: Ayush Singh

Problems/Data-structures/Day-09/sol/ayush2005k/Solution1.cpp

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+/*
+Problem: Salary Queries
+Source: CSES
+Problem Link: https://cses.fi/problemset/task/1144/
+
+Short Problem Statement:
+A company has n employees with salaries.
+You need to process two types of queries:
+1) Update an employee’s salary.
+2) Count how many employees have salaries in a given range [a, b].
+
+Approach:
+- Use coordinate compression because salaries can be up to 1e9.
+- Maintain frequencies of salaries using a Fenwick Tree (Binary Indexed Tree).
+- For update queries, decrease the old salary count and increase the new one.
+- For range queries, use prefix sums from the Fenwick Tree.
+
+Time Complexity:
+O((n + q) log (n + q))
+
+Space Complexity:
+O(n + q)
+
+Example:
+Input:
+5 3
+3 7 2 2 5
+? 2 3
+! 3 6
+? 2 3
+
+Output:
+3
+2
+
+Submission Link:
+https://cses.fi/paste/129b78ff774776c0f188fe/
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+struct Fenwick {
+    int n;
+    vector<int> bit;
+
+    Fenwick(int n) : n(n), bit(n + 1, 0) {}
+
+    void update(int i, int v) {
+        for (; i <= n; i += i & -i)
+            bit[i] += v;
+    }
+
+    int query(int i) {
+        int s = 0;
+        for (; i > 0; i -= i & -i)
+            s += bit[i];
+        return s;
+    }
+};
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    int n, q;
+    cin >> n >> q;
+
+    vector<int> salary(n);
+    for (int i = 0; i < n; i++)
+        cin >> salary[i];
+
+    vector<int> all = salary;
+    vector<pair<char, pair<int, int>>> queries;
+
+    for (int i = 0; i < q; i++) {
+        char type;
+        cin >> type;
+        if (type == '!') {
+            int k, x;
+            cin >> k >> x;
+            queries.push_back({type, {k - 1, x}});
+            all.push_back(x);
+        } else {
+            int a, b;
+            cin >> a >> b;
+            queries.push_back({type, {a, b}});
+            all.push_back(a);
+            all.push_back(b);
+        }
+    }
+
+    // Coordinate compression
+    sort(all.begin(), all.end());
+    all.erase(unique(all.begin(), all.end()), all.end());
+
+    auto get_id = [&](int x) {
+        return int(lower_bound(all.begin(), all.end(), x) - all.begin()) + 1;
+    };
+
+    Fenwick fw(all.size());
+
+    for (int x : salary)
+        fw.update(get_id(x), 1);
+
+    for (auto &qr : queries) {
+        if (qr.first == '!') {
+            int idx = qr.second.first;
+            int x = qr.second.second;
+
+            fw.update(get_id(salary[idx]), -1);
+            fw.update(get_id(x), 1);
+            salary[idx] = x;
+        } else {
+            int a = qr.second.first;
+            int b = qr.second.second;
+            cout << fw.query(get_id(b)) - fw.query(get_id(a) - 1) << "\n";
+        }
+    }
+
+    return 0;
+}