Add Problem Seven.

ProblemSeven.java

@@ -0,0 +1,45 @@
+/*
+ By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
+What is the 10 001st prime number? 
+ */
+public class ProblemSeven {
+
+	public static void main(String[] args) {
+		        int N = 1000000;
+
+		        // initially assume all integers are prime
+		        boolean[] isPrime = new boolean[N + 1];
+		        for (int i = 2; i <= N; i++) {
+		            isPrime[i] = true;
+		        }
+
+		        // mark non-primes <= N using Sieve of Erastothenes
+		        for (int i = 2; i*i <= N; i++) {
+
+		            // if i is prime, then mark multiples of i as nonprime
+		            // suffices to consider mutiples i, i+1, ..., N/i
+		            if (isPrime[i]) {
+		                for (int j = i; i*j <= N; j++) {
+		                    isPrime[i*j] = false;
+		                }
+		            }
+		        }
+
+		        // count primes
+		        int primes = 0;
+		        outerloop:
+		        for (int i = 2; i <= N; i++) {
+		            if (isPrime[i]){ primes++;
+		            if (primes==10001){
+		            System.out.println(i);
+		            break outerloop;
+		            }
+		            }
+		        }
+		        System.out.println("The number of primes <= " + N + " is " + primes);
+		    }
+		}
+
+
+
+