Euler 214

euler/e214.cpp

@@ -0,0 +1,24 @@
+#include <cstdio>
+#include <ctime>
+
+const int MAXP = 40000000, ROOT = 6325;
+
+int phi[MAXP];
+int chain[MAXP];
+int main(void){
+	clock_t begin = clock();
+	long long ans = 0;
+	chain[0] = chain[1] = 1;
+	for(int i = 0; i < MAXP; ++i) phi[i] = i;
+	for(int i = 2; i < MAXP; i += 2) phi[i] >>= 1;
+	for(int i = 3; i < MAXP; i += 2)
+		if(phi[i] == i) for(int j = i; j < MAXP; j += i)
+			phi[j] -= phi[j]/i;
+	for(int i = 2; i < MAXP; ++i)
+		if((chain[i] = 1 + chain[phi[i]]) == 25 && phi[i] == i - 1)
+			ans += i;
+	printf("Answer: %lld\n", ans);
+	printf("Time spent: %.3lfs\n", (double)(clock() - begin)/CLOCKS_PER_SEC);
+	return 0;
+}
+