Added Solution for Problem 3 in C++

Problems/Problem_3/Problem_3.cpp

@@ -0,0 +1,34 @@
+#include<iostream>
+
+using namespace std;
+
+bool isPrime(long long int N){
+    //2 is the only even prime
+    if(N==2 || N==3){
+        return true;
+    }
+    //Non-neighbours of 6 except 2 and 3 are non-primes
+    else if(N%6!=1 && N%6!=5){
+        return false;
+    }
+    //we only need to check if any odd number less
+    //than the square root of N divides N
+    for(long long int i=3; i*i <= N; i+=2){
+        if(N % i == 0){
+            return false;
+        }
+    }
+    return true;
+}
+
+int main(){
+    long long int N,i;
+    cin>>N;
+    for(i=2; N>0; i++) {
+        if(isPrime(i)) {
+            N--;
+        }
+    }
+    cout<<i-1;
+    return 0;
+}