204. Count Primes

Author: amitbansal7

count-primes.cpp

@@ -0,0 +1,61 @@
+//Runtime: 289 ms
+class Solution {
+public:
+
+    bool isprime(int n)
+    {
+        if(n<=1) return 0;
+
+        if(n<=3) return 1;
+
+        if(n%2==0 || n%3==0) return 0;
+
+        int  i =5;
+
+        while(i*i<=n)
+        {
+            if(n%i==0 || n%(i+2) == 0)
+                return 0;
+
+            i+=6;
+        }
+        return 1;
+    }
+
+    int countPrimes(int n) {
+        int cnt = 0;
+
+        for(int i=0;i<n;i++)
+            if(isprime(i))
+                cnt++;
+
+        return cnt;
+    }
+};
+
+//Runtime: 16 ms
+//sieve of eratosthenes
+class Solution {
+public:
+
+    int countPrimes(int n) {
+        int cnt = 0;
+
+        if(n<=2)
+            return 0;
+
+        bool S[n+1];
+        memset(S, true, sizeof(S));
+
+        for(int i=2;i<sqrt(n);i++)
+            if(S[i])
+                for(int j=i*i;j<=n;j+=i)
+                    S[j] = 0;
+
+        for(int i=2;i<n;i++)
+            if(S[i])
+                cnt++;
+
+        return cnt;
+    }
+};