Solved 72 with a good totient function I found online

projecteuler/072/main.cpp

@@ -0,0 +1,49 @@
+/*
+
+Counting Fractions
+
+Consider the fraction, n/d, where n and d are positive integers. If n<d and HCF(n,d)=1, it is called a reduced proper fraction.
+
+If we list the set of reduced proper fractions for d ≤ 8 in ascending order of size, we get:
+
+1/8, 1/7, 1/6, 1/5, 1/4, 2/7, 1/3, 3/8, 2/5, 3/7, 1/2, 4/7, 3/5, 5/8, 2/3, 5/7, 3/4, 4/5, 5/6, 6/7, 7/8
+
+It can be seen that there are 21 elements in this set.
+
+How many elements would be contained in the set of reduced proper fractions for d ≤ 1,000,000?
+
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+const int MAX = 1000000;
+
+int main(){
+
+    cout << "Hello this is Patrick" << endl;
+    auto start = chrono::high_resolution_clock::now();
+
+    vector<int> phi(MAX + 1);
+    for(int i = 2; i <= MAX; ++i){
+        phi[i] = i;
+    }
+
+    long long sum = 0;
+
+    for(int i = 2; i <= MAX; ++i){
+        if(phi[i] == i){
+            for(int j = i; j <= MAX; j += i){
+                phi[j] = phi[j] / i * (i - 1);
+            }
+        }
+        sum += phi[i];
+    }
+
+    cout << sum << endl;
+
+    auto duration = chrono::duration_cast<chrono::milliseconds>(chrono::high_resolution_clock::now() - start);
+    cout << (float)duration.count()/1000 << endl;
+    return 0;
+}