solutions

Author: liuhanqu

algorithms/easy/countPrimes/countPrimes.js

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+// description: https://leetcode-cn.com/explore/interview/card/top-interview-questions-easy/25/math/61/
+// 计数质数
+
+function isPrime(n) {
+  if (n <= 1) {
+    return false;
+  }
+  for (var i = 2; i * i <= n; i++) {
+    if (n % i === 0) return false;
+  }
+  return true;
+}
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var countPrimes = function(n) {
+  var count = 0;
+  for (var i = 1; i < n; i++) {
+    if (isPrime(i)) {
+      count = count + 1;
+    }
+  }
+  return count;
+};

algorithms/easy/isPowerOfThree/isPowerOfThree.js

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+// description: https://leetcode-cn.com/explore/interview/card/top-interview-questions-easy/25/math/62/
+// 3的幂
+
+/**
+ * @param {number} n
+ * @return {boolean}
+ */
+var isPowerOfThree = function(n) {
+  if (n === 1) {
+    return true;
+  }
+  if (n % 3 !== 0 || n === 0) {
+    return false;
+  }
+
+  return isPowerOfThree(n / 3);
+};