Using Constructors and Comparators in C++

First I will describe using constructors in C++.

struct point {
    int x, y;
    point() {} // default constructor
    point (int _x, int _y) {
        x = _x;
        y = _y;
    }
};
// Note that in C++, You do not need to use typedef as it is already typedefed
to point, So you do need to use
// typedef for that.

Another way

struct point {
    int x, y;
    point() {} // default constructor
    point (int x, int y): x(x), y(y) {}
};

Now using comparators in C++

struct point {
    int x, y;
    point() {}
    point (int x, int y) : x(x), y(y) {}
    
    // overloading of < operator
    bool operator<(const point &rhs) const{
        // your main logic for the comparator goes here
        return make_pair(y,x) < make_pair(rhs.y, rhs.x);
    }

For more information about using operator overloading you can visit this link
http://www.cprogramming.com/tutorial/operator_overloading.html
Final Code:

// Comment
struct point {
    int x, y;
    point() {}
    point (int _x, int _y) {
        x = _x;
        y = _y;
    }
    bool operator<(const point &rhs) const{
        return make_pair(y,x) < make_pair(rhs.y, rhs.x);
    }
    bool operator==(const point &rhs) const{
        return make_pair(y,x) == make_pair(rhs.y, rhs.x);
    }
};

Sample uses:
 sorting an array in reverse order
 Method 1:

vector a;
sort(a.rbegin(), a.rend());

Method 2:

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

For an vector

#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

bool cmp(int a,int b) {
    return a >= b;
}

int main() {
    int n;
    cin >> n;

    vector a;

    for (int i = 0; i < n; i++) {
        int x;
        cin >> x;
        a.push_back(x);
    }

    sort (a.begin(), a.end(), cmp);

    for (int i = 0; i < a.size(); i++)
        cout << a[i] << " ";


    return 0;
}

For an array now.

#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

bool cmp(int a,int b) {
    return a >= b;
}

int a[100];

int main() {
    int n;
    cin >> n;

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

    sort (a, a + n, cmp);

    for (int i = 0; i < a.size(); i++)
        cout << a[i] << " ";


    return 0;
}

Deleting redundent points out of n points given. Two points are considered to be equal if both their x and y coordinates are equal.

#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

struct point {
    int x, y;
    point() {}
    point (int x, int y) : x(x), y(y) {}
    bool operator<(const point &rhs) const{
        return make_pair(y,x) < make_pair(rhs.y, rhs.x);
    }
    bool operator==(const point &rhs) const{
        return make_pair(y,x) == make_pair(rhs.y, rhs.x);
    }
};

int main() {
    int n;
    cin >> n;

    vector a;

    for (int i = 0; i < n; i++) {
        int x, y;
        cin >> x >> y;
        a.push_back(point(x, y));
    }

    set st;

    for (int i = 0; i < n; i++) {
        st.insert(a[i]);
    }

    vector res;
    set :: iterator it;
    for (it = st.begin(); it != st.end(); it++) {
        res.push_back(*it);
    }

    for (int i = 0; i < res.size(); i++)
        cout << res[i].x << "  " << res[i].y << endl;

    return 0;
}