Create leetcode204.cpp

C++/leetcode204.cpp

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+/*
+204. Count Primes
+Given an integer n, return the number of prime numbers that are strictly less than n.
+Example 1:
+Input: n = 10
+Output: 4
+Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
+*/
+class Solution {
+public:
+    int countPrimes(int n) {
+        if(n == 0)
+            return 0;
+        vector<bool> prime(n,true);
+        prime[0] = prime [1] = false ;
+        int ans = 0;
+        for(int i = 2 ; i<n ; i++)
+        {
+            if(prime[i])
+            {
+                ans++;
+                int j=2*i;
+                while(j<n)
+                {
+                    prime[j] = false;
+                    j = j + i ;
+                }
+            }
+        }
+        return ans;
+    }
+};