C++ solution of problem 35.

35/euler35.cpp

@@ -0,0 +1,70 @@
+#include <cmath>
+#include <vector>
+#include <algorithm>
+#include <iostream>
+
+void sieve(std::vector<int>& primes, int const limit) {
+    if (limit < 2) {
+        return;
+    } else if (limit < 3) {
+        primes.push_back(2);
+        return;
+    }
+    primes.push_back(2);
+    primes.push_back(3);
+
+    for (int num = 5; num <= limit; num += 2) {
+        int sqrtnum = (int) std::sqrt(num);
+        bool isPrime = true;
+        for (auto primeIt = ++primes.begin(); primeIt != primes.end() && *primeIt <= sqrtnum; ++primeIt) {
+            if (num % *primeIt == 0) {
+                isPrime = false;
+                break;
+            }
+        }
+        if (isPrime) {
+            primes.push_back(num);
+        }
+    }
+}
+
+int main(void) {
+    constexpr int limit = 1000000;
+    constexpr int PRIMELIMIT = limit - 1;
+    std::vector<int> primes;
+    sieve(primes, PRIMELIMIT);
+
+    std::vector<int> circular;
+    auto primeIt = primes.begin();
+    while (*primeIt < 10) {
+        circular.push_back(*primeIt);
+        ++primeIt;
+    }
+    int tens = 10;
+    int nextTens = 100;
+    int rotations = 2;
+
+    while (primeIt != primes.end()) {
+        if (*primeIt >= nextTens) {
+            tens *= 10;
+            nextTens *= 10;
+            ++rotations;
+        }
+
+        bool isCircular = true;
+        int prime = *primeIt;
+        for (int i = 1; i < rotations; ++i) {
+            prime = prime / 10 + (prime % 10) * tens;
+            isCircular = std::binary_search(primes.begin(), primes.end(), prime)? true: false;
+            if (!isCircular) {
+                break;
+            }
+        }
+        if (isCircular) {
+            circular.push_back(*primeIt);
+        }
+        ++primeIt;
+    }
+
+    std::cout << circular.size() << std::endl;
+}