two-sum.cpp

#include <iostream>
#include <algorithm>

using namespace std;

int find(int k, int* a, int start, int end)
{
    while (start <= end) {
        int mid = start + (end-start) / 2;
        if (a[mid] < k) start = mid + 1; // re phai
        else if (a[mid] > k) end = mid - 1; // re trai
        else return mid;
    }
    return -1;
}

int main()
{
    int n;
    int a[10000];
    cin >> n;
    for (int i = 0; i < n; i++) cin >> a[i];

    sort(a, a+n);

    for (int i=0; i< n; i++) {
        int j = find(-a[i], a, i+1, n-1);  // -1 if not found
        if (j>0) cout << a[i] << " " << a[j] << endl;
    }


    /*
    for (int i=0; i< n; i++)
        for (int j = i+1; j <n; j++)
            if (a[i] == -a[j]) cout << a[i] << " " << a[j] << endl;
                */
}