二分探索を使う問題(https://atcoder.jp/contests/abc077/tasks/arc084_a , https://atcoder.jp/contests/abc143/tasks/abc143_d )のソースコード

abc077c.cpp

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

int main()
{
    vector<int> a, b, c;
    int n, inp;
    long long int ans = 0;
    cin >> n;

    for(int i = 0; i < n; i++){
        cin >> inp;
        a.push_back(inp);
    }
    sort(a.begin(), a.end());

    for(int i = 0; i < n; i++){
        cin >> inp;
        b.push_back(inp);
    }
    sort(b.begin(), b.end());

    for(int i = 0; i < n; i++){
        cin >> inp;
        c.push_back(inp);
    }
    sort(c.begin(), c.end());

    for(int i = 0; i < n; i++){
        long long int ai = distance(a.begin(), lower_bound(a.begin(), a.end(), b[i]));
        long long int ci = distance(upper_bound(c.begin(), c.end(), b[i]), c.end());
        ans += ai * ci;
    }

    cout << ans << endl;
    return 0;
}

abc143d.cpp

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

int main()
{
    int n, li, ans = 0;
    cin >> n;
    vector<int> l;
    for(int i = 0; i < n; i++){
        cin >> li;
        l.push_back(li);
    }
    sort(l.begin(), l.end());

    for(int m1 = n - 1; m1 >= 2; m1--){
        for(int m2 = m1 - 1; m2 >= 1; m2--){
            ans += distance(upper_bound(l.begin(), l.begin() + m2, l[m1] - l[m2]), l.begin() + m2);
        }
    }
    cout << ans << endl;
}