C++ for competitive programming: fast I/O, STL containers and algorithms, complexity, common patterns and how to practise

#include <bits/stdc++.h>          // includes the whole standard library (GCC-specific, contests only)
using namespace std;
using ll = long long;

int main() {
    ios::sync_with_stdio(false);  // fast I/O
    cin.tie(nullptr);

    int t;                        // many problems have several test cases
    cin >> t;
    while (t--) {
        int n;
        cin >> n;
        vector<ll> a(n);
        for (auto &x : a) cin >> x;
        cout << accumulate(a.begin(), a.end(), 0LL) << "\n";   // "\n" is faster than endl
    }
    return 0;
}

#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <string>
using namespace std;

int main() {
    vector<int> v = {5, 3, 8, 1};
    v.push_back(10);
    cout << "size " << v.size() << ", last " << v.back() << endl;

    pair<string, int> p = {"Nairobi", 47};
    cout << p.first << " " << p.second << endl;

    set<int> s = {5, 1, 5, 3};                 // sorted, unique
    for (int x : s) cout << x << " ";
    cout << "| has 3? " << s.count(3) << endl;

    map<string, int> stock;                    // sorted keys
    stock["unga"] = 40;
    stock["sugar"] = 25;
    stock["unga"] -= 5;
    for (auto &item : stock) cout << item.first << "=" << item.second << " ";
    cout << endl;
    return 0;
}

#include <iostream>
#include <queue>
#include <stack>
#include <vector>
using namespace std;

int main() {
    priority_queue<int> maxHeap;                               // largest on top
    priority_queue<int, vector<int>, greater<int>> minHeap;    // smallest on top
    int vals[5] = {7, 2, 9, 4, 1};
    for (int i = 0; i < 5; i++) {
        maxHeap.push(vals[i]);
        minHeap.push(vals[i]);
    }
    cout << "max " << maxHeap.top() << ", min " << minHeap.top() << endl;

    stack<char> st;
    string brackets = "(()())";
    bool ok = true;
    for (char c : brackets) {
        if (c == '(') st.push(c);
        else if (st.empty()) { ok = false; break; }
        else st.pop();
    }
    cout << "Balanced: " << (ok && st.empty() ? "yes" : "no") << endl;
    return 0;
}

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <string>
using namespace std;

int main() {
    vector<int> marks = {67, 82, 45, 90, 58, 82};
    sort(marks.begin(), marks.end());                       // ascending
    for (int m : marks) cout << m << " ";
    cout << endl;

    sort(marks.begin(), marks.end(), greater<int>());       // descending
    cout << "Top mark: " << marks[0] << endl;

    cout << "Sum: " << accumulate(marks.begin(), marks.end(), 0) << endl;
    cout << "Max: " << *max_element(marks.begin(), marks.end()) << endl;
    cout << "Count of 82: " << count(marks.begin(), marks.end(), 82) << endl;

    sort(marks.begin(), marks.end());
    cout << "Has 58 (binary search)? " << binary_search(marks.begin(), marks.end(), 58) << endl;
    cout << "First >= 60 at index " << (lower_bound(marks.begin(), marks.end(), 60) - marks.begin()) << endl;

    vector<pair<string, int>> students = {{"Juma", 70}, {"Amina", 91}, {"Brian", 70}};
    sort(students.begin(), students.end(), [](const pair<string, int> &a, const pair<string, int> &b) {
        if (a.second != b.second) return a.second > b.second;   // higher mark first
        return a.first < b.first;                               // then by name
    });
    for (auto &s : students) cout << s.first << " " << s.second << endl;
    return 0;
}

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<long long> sales = {120, 450, 80, 300, 220, 510};
    int n = sales.size();
    vector<long long> prefix(n + 1, 0);
    for (int i = 0; i < n; i++) prefix[i + 1] = prefix[i] + sales[i];

    // total sales for days 2..4 (0-based, inclusive)
    int l = 2, r = 4;
    cout << "Days " << l << "-" << r << ": " << prefix[r + 1] - prefix[l] << endl;
    return 0;
}

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    vector<int> prices = {800, 1200, 900, 2500, 3500, 400};
    int budget = 3300;
    sort(prices.begin(), prices.end());
    int i = 0, j = prices.size() - 1, best = -1;
    while (i < j) {
        int sum = prices[i] + prices[j];
        if (sum <= budget) { best = max(best, sum); i++; }
        else j--;
    }
    cout << "Best pair total within budget: " << best << endl;
    return 0;
}

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    string votes = "ABACABBCA";
    map<char, int> freq;
    for (char c : votes) freq[c]++;
    for (auto &f : freq) cout << f.first << ": " << f.second << endl;
    return 0;
}