Added happy number program.

Happy_number.cpp

@@ -0,0 +1,26 @@
+/* A happy number is a number whose sum of digits is calculated until the sum is a single digit,
+  and this sum turns out to be 1 */
+#include <iostream>
+using namespace std;
+int main()
+{
+  int n,k,s=0,d;
+  cout << "Enter a number:";
+  cin >> n;
+  s=n;k=n;
+  while(k>9)
+  {
+    while(k!=0)
+    {
+      d=n%10;
+      s+=d;
+      n/=10;
+    }
+    k=s;
+    s=0;
+  }
+  if(k==1)
+   cout << n << " is a happy number" << endl;
+  else
+   cout << n << " is not a happy number" << endl;
+}