first n primes

Author: JoshuaJiangFD

src/main/java/joshua/algorithm/math/FirstNPrime.java

@@ -0,0 +1,47 @@
+package joshua.algorithm.math;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * 输出前 N 个 质数，质数除1外只能被自身整除。
+ *
+ *
+ * Created by joshu on 2016/5/25.
+ */
+public abstract class FirstNPrime {
+
+    public abstract List<Long> getFirstNPrime(int n);
+
+    /**
+     * 类似 {@link joshua.leetcode.math.CountPrime} 的 方法。
+     *  对每个数m，已知 小于m的所有的质数[2,..k,..]，则对于小于m的开方的所有质数，如果都不能整除m，则m也是素数
+     *  否则继续对m+1判断。
+     */
+    public static class Solution1 extends FirstNPrime {
+
+        @Override
+        public List<Long> getFirstNPrime(int n) {
+            List<Long> primes = new ArrayList<Long>();
+            primes.add(2L);
+            int count = 1;
+            long currentNum = 3;
+            while (count < n) {
+                int k = 0;
+                // get the next prime, count == primes.size()
+                while (k < count && primes.get(k) * primes.get(k) <= currentNum) {
+                    if (currentNum % primes.get(k) == 0) {
+                        k = 0;
+                        currentNum +=1;
+                    } else {
+                        k++;
+                    }
+                }
+                primes.add(currentNum);
+                currentNum += 1;
+                count++;
+            }
+            return primes;
+        }
+    }
+}

src/test/java/joshua/algorithm/math/FirstNPrimeTest.java

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+package joshua.algorithm.math;
+
+import com.google.common.collect.Lists;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+/**
+ * Created by joshu on 2016/5/25.
+ */
+public class FirstNPrimeTest {
+
+    @Test
+    public void testSolution1() {
+        FirstNPrime solution = new FirstNPrime.Solution1();
+        assertEquals(Lists.newArrayList(2L,3L,5L,7L,11L), solution.getFirstNPrime(5));
+    }
+}