Added Dutch Flag algo (#87)

arrays/README.md

@@ -3,6 +3,7 @@
 ### C or C++
 
 1. [Counting Inversions](c-or-cpp/count-inversions.cpp)
+2. [Dutch Flag Algo](c-or-cpp/dutch-flag-algo.cpp)
 
 ### Python
 

arrays/c-or-cpp/dutch-flag-algo.cpp

@@ -0,0 +1,53 @@
+/*  Dutch Flag Algo: To separate three different entities
+    e.g :- Sort 0s,1s and 2s in an array without using sorting algo
+    Complexity : O(n)  
+*/
+
+#include <iostream>
+using namespace std;
+
+/*  0 0 0 1 1 1 ? ? ? ? 2 2 2
+          |     |     |
+          v     v     v  
+         Low   Mid   High
+   
+   > In this algo, we tend to shrink this '?' region 
+   whose ends are pointed by Mid and High indexes.
+
+   > Low points to the region where the 1s start
+
+*/
+
+void dutch_flag(int arr[],int size){
+    //mid to high is the region we are about to shrink
+    int low = 0, mid = 0, high = size-1;
+    while(mid <= high){
+        if(arr[mid] == 0){
+            swap(arr[low],arr[mid]);
+            low++; mid++;
+        }
+        else if(arr[mid] == 1){
+            mid++;
+        }
+        else if(arr[mid] == 2){
+            swap(arr[high],arr[mid]);
+            high--;
+        }
+    }
+}
+
+//Utility function to print the array
+void printArray(int arr[], int arr_size) 
+{  
+    for (int i = 0; i < arr_size; i++) 
+        cout << arr[i] << " "; 
+} 
+
+
+int main(){
+    int a[] = {0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1};
+    int n = sizeof(a)/sizeof(a[0]);
+    dutch_flag(a,n);
+    printArray(a,n);
+    return 0;
+}