implemented graham scan in C plus plus (#616)

* implemented graham scan in cpp

* improved code for graham-scan implementation in cpp

* improved implementation based on suggestions

* fixed the runtimeerror

* fixed runtime error based on suggestion

Author: AkashDhiman

CONTRIBUTORS.md

@@ -45,3 +45,4 @@ This file lists everyone, who contributed to this repo and wanted to show up her
 - Olav Sundfør
 - dovisutu
 - Antetokounpo
+- Akash Dhiman

contents/graham_scan/code/c++/graham_scan.cpp

@@ -0,0 +1,65 @@
+#include <iostream>
+#include <vector>
+#include <algorithm>
+
+struct point{
+  double x;
+  double y;
+};
+
+bool ccw(point a, point b, point c){
+  return ((b.x - a.x)*(c.y - a.y) > (b.y - a.y)*(c.x - a.x));
+}
+
+std::vector<point> grahamScan(std::vector<point> points){
+  //selecting lowest point as pivot
+  int lowIndex = 0;
+  for(size_t i = 1; i < points.size(); i++) {
+    if(points[i].y < points[lowIndex].y){
+      lowIndex = i;
+    }
+  }
+  std::swap(points[0], points[lowIndex]);
+  point pivot = points[0];
+
+  //sorting points by polar angle
+  std::sort(points.begin()+1, points.end(), [pivot](point a, point b){
+    return ccw(pivot, a, b);
+  });
+
+  //creating convex hull
+  size_t m = 1;
+  for(size_t i = 2; i < points.size(); i++) {
+    while(ccw(points[m - 1], points[m], points[i]) <= 0){
+      if(m > 1) {
+                m--;
+                continue;
+            } else if(i == points.size()) {
+                break;
+            } else {
+                i++;
+            }
+    }
+    m++;
+    std::swap(points[i], points[m]);
+  }
+  return std::vector<point>(points.begin(), points.begin() + m + 1);
+}
+
+void print(std::vector<point> points){
+  for ( auto p : points){
+    std::cout <<"(" << p.x << ", " << p.y <<")\n";
+  }
+}
+
+int main(){
+  std::vector<point> points = {{-5, 2}, {5, 7}, {-6, -12}, {-14, -14}, {9, 9},
+                              {-1, -1}, {-10, 11}, {-6, 15}, {-6, -8}, {15, -9},
+                              {7, -7}, {-2, -9}, {6, -5}, {0, 14}, {2, 8}};
+  std::cout << "original points are as follows:\n";
+  print(points);
+  std::vector<point> hull = grahamScan(points);
+  std::cout << "points in hull are as follows:\n";
+  print(hull);
+  return 0;
+}

contents/graham_scan/graham_scan.md

@@ -26,6 +26,8 @@ We can find whether a rotation is counter-clockwise with trigonometric functions
 [import:13-15, lang:"go"](code/golang/graham.go)
 {% sample lang="java" %}
 [import:27-29, lang:"java"](code/java/GrahamScan.java)
+{% sample lang="cpp" %}
+[import:10-12, lang="cpp"](code/c++/graham_scan.cpp)
 {% endmethod %}
 
 If the output of this function is 0, the points are collinear.
@@ -54,6 +56,8 @@ In the end, the code should look something like this:
 [import:21-42, lang:"go"](code/golang/graham.go)
 {% sample lang="java" %}
 [import:35-70, lang:"java"](code/java/GrahamScan.java)
+{% sample lang="cpp" %}
+[import:14-47, lang="cpp"](code/c++/graham_scan.cpp)
 {% endmethod %}
 
 ### Bibliography
@@ -77,6 +81,8 @@ In the end, the code should look something like this:
 [import, lang:"go"](code/golang/graham.go)
 {% sample lang="java" %}
 [import, lang:"java"](code/java/GrahamScan.java)
+{% sample lang="cpp" %}
+[import, lang="cpp"](code/c++/graham_scan.cpp)
 {% endmethod %}
 
 <script>