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Deleting points from Convex Hull
• Difficulty Level : Medium
• Last Updated : 11 Jun, 2017

Given a fixed set of points. We need to find convex hull of given set. We also need to find convex hull when a point is removed from the set.

Example:

```Initial Set of Points: (-2, 8) (-1, 2) (0, 1) (1, 0)
(-3, 0) (-1, -9) (2, -6) (3, 0)
(5, 3) (2, 5)
Initial convex hull:- (-2, 8) (-3, 0) (-1, -9) (2, -6)
(5, 3)
Point to remove from the set : (-2, 8)
Final convex hull: (2, 5) (-3, 0) (-1, -9) (2, -6) (5, 3)
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Prerequisite : Convex Hull (Simple Divide and Conquer Algorithm)

The algorithm for solving the above problem is very easy. We simply check whether the point to be removed is a part of the convex hull. If it is, then we have to remove that point from the initial set and then make the convex hull again (refer Convex hull (divide and conquer) ).
And if not then we already have the solution (the convex hull will not change).

 `// C++ program to demonstrate delete operation``// on Convex Hull.``#include``using` `namespace` `std;`` ` `// stores the center of polygon (It is made``// global becuase it is used in comare function)``pair<``int``, ``int``> mid;`` ` `// determines the quadrant of a point``// (used in compare())``int` `quad(pair<``int``, ``int``> p)``{``    ``if` `(p.first >= 0 && p.second >= 0)``        ``return` `1;``    ``if` `(p.first <= 0 && p.second >= 0)``        ``return` `2;``    ``if` `(p.first <= 0 && p.second <= 0)``        ``return` `3;``    ``return` `4;``}`` ` `// Checks whether the line is crossing the polygon``int` `orientation(pair<``int``, ``int``> a, pair<``int``, ``int``> b,``                ``pair<``int``, ``int``> c)``{``    ``int` `res = (b.second-a.second)*(c.first-b.first) -``              ``(c.second-b.second)*(b.first-a.first);`` ` `    ``if` `(res == 0)``        ``return` `0;``    ``if` `(res > 0)``        ``return` `1;``    ``return` `-1;``}`` ` `// compare function for sorting``bool` `compare(pair<``int``, ``int``> p1, pair<``int``, ``int``> q1)``{``    ``pair<``int``, ``int``> p = make_pair(p1.first - mid.first,``                                 ``p1.second - mid.second);``    ``pair<``int``, ``int``> q = make_pair(q1.first - mid.first,``                                 ``q1.second - mid.second);`` ` `    ``int` `one = quad(p);``    ``int` `two = quad(q);`` ` `    ``if` `(one != two)``        ``return` `(one < two);``    ``return` `(p.second*q.first < q.second*p.first);``}`` ` `// Finds upper tangent of two polygons 'a' and 'b'``// represented as two vectors.``vector > merger(vector > a,``                               ``vector > b)``{``    ``// n1 -> number of points in polygon a``    ``// n2 -> number of points in polygon b``    ``int` `n1 = a.size(), n2 = b.size();`` ` `    ``int` `ia = 0, ib = 0;``    ``for` `(``int` `i=1; i a[ia].first)``            ``ia = i;`` ` `    ``// ib -> leftmost point of b``    ``for` `(``int` `i=1; i=0)``            ``inda = (inda + 1) % n1;`` ` `        ``while` `(orientation(a[inda], b[indb], b[(n2+indb-1)%n2]) <=0)``        ``{``            ``indb = (n2+indb-1)%n2;``            ``done = 0;``        ``}``    ``}`` ` `    ``int` `uppera = inda, upperb = indb;``    ``inda = ia, indb=ib;``    ``done = 0;``    ``int` `g = 0;``    ``while` `(!done)``//finding the lower tangent``    ``{``        ``done = 1;``        ``while` `(orientation(a[inda], b[indb], b[(indb+1)%n2])>=0)``            ``indb=(indb+1)%n2;`` ` `        ``while` `(orientation(b[indb], a[inda], a[(n1+inda-1)%n1])<=0)``        ``{``            ``inda=(n1+inda-1)%n1;``            ``done=0;``        ``}``    ``}`` ` `    ``int` `lowera = inda, lowerb = indb;``    ``vector > ret;`` ` `    ``//ret contains the convex hull after merging the two convex hulls``    ``//with the points sorted in anti-clockwise order``    ``int` `ind = uppera;``    ``ret.push_back(a[uppera]);``    ``while` `(ind != lowera)``    ``{``        ``ind = (ind+1)%n1;``        ``ret.push_back(a[ind]);``    ``}`` ` `    ``ind = lowerb;``    ``ret.push_back(b[lowerb]);``    ``while` `(ind != upperb)``    ``{``        ``ind = (ind+1)%n2;``        ``ret.push_back(b[ind]);``    ``}``    ``return` `ret;`` ` `}`` ` `// Brute force algorithm to find convex hull for a set``// of less than 6 points``vector > bruteHull(vector > a)``{``    ``// Take any pair of points from the set and check``    ``// whether it is the edge of the convex hull or not.``    ``// if all the remaining points are on the same side``    ``// of the line then the line is the edge of convex``    ``// hull otherwise not``    ``set >s;`` ` `    ``for` `(``int` `i=0; i= 0)``                    ``pos++;``            ``}``            ``if` `(pos == a.size() || neg == a.size())``            ``{``                ``s.insert(a[i]);``                ``s.insert(a[j]);``            ``}``        ``}``    ``}`` ` `    ``vector >ret;``    ``for` `(``auto` `e : s)``        ``ret.push_back(e);`` ` `    ``// Sorting the points in the anti-clockwise order``    ``mid = {0, 0};``    ``int` `n = ret.size();``    ``for` `(``int` `i=0; i> findHull(vector> a)``{``    ``// If the number of points is less than 6 then the``    ``// function uses the brute algorithm to find the``    ``// convex hull``    ``if` `(a.size() <= 5)``        ``return` `bruteHull(a);`` ` `    ``// left contains the left half points``    ``// right contains the right half points``    ``vector>left, right;``    ``for` `(``int` `i=0; i>left_hull = findHull(left);``    ``vector>right_hull = findHull(right);`` ` `    ``// merging the convex hulls``    ``return` `merger(left_hull, right_hull);``}`` ` `// Returns the convex hull for the given set of points after``// remviubg a point p.``vector> removePoint(vector> a,``                                   ``vector> hull,``                                   ``pair<``int``, ``int``> p)``{``    ``// checking whether the point is a part of the``    ``// convex hull or not.``    ``bool` `found = 0;``    ``for` `(``int` `i=0; i < hull.size() && !found; i++)``        ``if` `(hull[i].first == p.first &&``                ``hull[i].second == p.second)``            ``found = 1;`` ` `    ``// If point is not part of convex hull``    ``if` `(found == 0)``        ``return` `hull;`` ` `    ``// if it is the part of the convex hull then``    ``// we remove the point and again make the convex hull``    ``// and if not, we print the same convex hull.``    ``for` `(``int` `i=0; i > a;``    ``a.push_back(make_pair(0, 0));``    ``a.push_back(make_pair(1, -4));``    ``a.push_back(make_pair(-1, -5));``    ``a.push_back(make_pair(-5, -3));``    ``a.push_back(make_pair(-3, -1));``    ``a.push_back(make_pair(-1, -3));``    ``a.push_back(make_pair(-2, -2));``    ``a.push_back(make_pair(-1, -1));``    ``a.push_back(make_pair(-2, -1));``    ``a.push_back(make_pair(-1, 1));`` ` `    ``int` `n = a.size();`` ` `    ``// sorting the set of points according``    ``// to the x-coordinate``    ``sort(a.begin(), a.end());``    ``vector >hull = findHull(a);`` ` `    ``cout << ``"Convex hull:\n"``;``    ``for` `(``auto` `e : hull)``        ``cout << e.first << ``" "``             ``<< e.second << endl;`` ` `    ``pair<``int``, ``int``> p = make_pair(-5, -3);``    ``removePoint(a, hull, p);`` ` `    ``cout << ``"\nModified Convex Hull:\n"``;``    ``for` `(``auto` `e:hull)``        ``cout << e.first << ``" "``             ``<< e.second << endl;`` ` `    ``return` `0;``}`

Output:

```convex hull:
-3 0
-1 -9
2 -6
5 3
2 5
```

Time Complexity:
It is simple to see that the maximum time taken per query is the time taken to construct the convex hull which is O(n*logn). So, the overall complexity is O(q*n*logn), where q is the number of points to be deleted.

This article is contributed by Amritya Vagmi. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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