1. What I learned

    a. Vector<>

        Vector is simply an array of dynamically changing lengths.

    b. sort(Vector<>)

        To sort the internal values of vectors in ascending order, sort() in the ‘algorithm’ header file can be used.

2. Code

#include <stdio.h>
#include <vector>
#include <algorithm>

using namespace std;

int main(void) {
    int num_of_points, x, y;
    vector<pair<int,int> > points;

    scanf("%d", &num_of_points);
    for (int i=0 ; i<num_of_points ; i++) {
        scanf("%d %d", &x, &y);
        points.push_back(pair<int,int>(y,x));
    }

    sort(points.begin(), points.end());

    for (int i=0 ; i<num_of_points ; i++) {
        printf("%d %d\n", points[i].second, points[i].first);
    }
    return 0;
}

3. Result

        Runtime : 64 ms, Memory usage : 2776 KB
        (Runtime can be different by a system even if it is a same code.)

Check out the my GitHub repo for more info on the code. If you have questions, you can leave a reply on this post.