Find Simple Closed Path for a given set of points

Given a set of points, connect the dots without crossing. 

Example: 

Input: points[] = {(0, 3), (1, 1), (2, 2), (4, 4),
                   (0, 0), (1, 2), (3, 1}, {3, 3}};

Output: Connecting points in following order would
        not cause any crossing
       {(0, 0), (3, 1), (1, 1), (2, 2), (3, 3),
        (4, 4), (1, 2), (0, 3)}

We strongly recommend you to minimize your browser and try this yourself first.
The idea is to use sorting. 

• Find the bottom-most point by comparing y coordinate of all points. If there are two points with same y value, then the point with smaller x coordinate value is considered. Put the bottom-most point at first position. 
  

• Consider the remaining n-1 points and sort them by polar angle in counterclockwise order around points[0]. If polar angle of two points is same, then put the nearest point first.
• Traversing the sorted array (sorted in increasing order of angle) yields simple closed path. 
  

How to compute angles?
One solution is to use trigonometric functions. 
Observation: We don’t care about the actual values of the angles. We just want to sort by angle. 
Idea: Use the orientation to compare angles without actually computing them!

Below is C++ implementation of above idea.  

C++

// A C++ program to find simple closed path for n points
// for explanation of orientation()
#include <bits/stdc++.h>
using namespace std;

struct Point
{
    int x, y;
};

// A global point needed for  sorting points with reference
// to the first point. Used in compare function of qsort()
Point p0;

// A utility function to swap two points
int swap(Point &p1, Point &p2)
{
    Point temp = p1;
    p1 = p2;
    p2 = temp;
}

// A utility function to return square of distance between
// p1 and p2
int dist(Point p1, Point p2)
{
    return (p1.x - p2.x)*(p1.x - p2.x) +
           (p1.y - p2.y)*(p1.y - p2.y);
}

// To find orientation of ordered triplet (p, q, r).
// The function returns following values
// 0 --> p, q and r are collinear
// 1 --> Clockwise
// 2 --> Counterclockwise
int orientation(Point p, Point q, Point r)
{
    int val = (q.y - p.y) * (r.x - q.x) -
              (q.x - p.x) * (r.y - q.y);

    if (val == 0) return 0;  // collinear
    return (val > 0)? 1: 2; // clockwise or counterclock wise
}

// A function used by library function qsort() to sort
//  an array of points with respect to the first point
int compare(const void *vp1, const void *vp2)
{
   Point *p1 = (Point *)vp1;
   Point *p2 = (Point *)vp2;

   // Find orientation
   int o = orientation(p0, *p1, *p2);
   if (o == 0)
     return (dist(p0, *p2) >= dist(p0, *p1))? -1 : 1;

   return (o == 2)? -1: 1;
}

// Prints simple closed path for a set of n points.
void printClosedPath(Point points[], int n)
{
   // Find the bottommost point
   int ymin = points[0].y, min = 0;
   for (int i = 1; i < n; i++)
   {
     int y = points[i].y;

     // Pick the bottom-most. In case of tie, choose the
     // left most point
     if ((y < ymin) || (ymin == y &&
         points[i].x < points[min].x))
        ymin = points[i].y, min = i;
   }

   // Place the bottom-most point at first position
   swap(points[0], points[min]);

   // Sort n-1 points with respect to the first point.
   // A point p1 comes before p2 in sorted output if p2
   // has larger polar angle (in counterclockwise
   // direction) than p1
   p0 = points[0];
   qsort(&points[1], n-1, sizeof(Point), compare);

   // Now stack has the output points, print contents
   // of stack
   for (int i=0; i<n; i++)
       cout << "(" << points[i].x << ", "
            << points[i].y <<"), ";
}

// Driver program to test above functions
int main()
{
    Point points[] = {{0, 3}, {1, 1}, {2, 2}, {4, 4},
                       {0, 0}, {1, 2}, {3, 1}, {3, 3}};
    int n = sizeof(points)/sizeof(points[0]);
    printClosedPath(points, n);
    return 0;
}

Output: 

(0, 0), (3, 1), (1, 1), (2, 2), (3, 3),
(4, 4), (1, 2), (0, 3), 

Time complexity of above solution is O(n Log n) if we use a O(nLogn) sorting algorithm for sorting points.
Auxiliary Space: O(1), since no extra space has been taken.

Source: 
http://www.dcs.gla.ac.uk/~pat/52233/slides/Geometry1x1.pdf
This article is contributed by Aarti_Rathi and Rajeev Agrawal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above