Update Primes.cpp

Author: wisdompeak

Template/Math/Primes.cpp

@@ -20,3 +20,17 @@ vector<int>Eratosthenes(int n)
     }
     return primes;
 }
+
+// Find the smallest prime factor for each element
+vector<int>spf(int N) {
+    vector<int>spf(N+1);
+    for (int i = 0; i <= N; ++i) spf[i] = i;
+    for (int i = 2; i * i <= maxA; ++i) {
+        if (spf[i] == i) {
+            for (int j = i * i; j <= N; j += i) {
+                if (spf[j] == j) spf[j] = i;
+            }
+        }
+    }    
+    return spf;
+}