diff --git a/src/main/java/g0201_0300/s0207_course_schedule/Solution.java b/src/main/java/g0201_0300/s0207_course_schedule/Solution.java
new file mode 100644
index 000000000..b217c9b24
--- /dev/null
+++ b/src/main/java/g0201_0300/s0207_course_schedule/Solution.java
@@ -0,0 +1,41 @@
+package g0201_0300.s0207_course_schedule;
+
+import java.util.ArrayList;
+
+@SuppressWarnings("unchecked")
+public class Solution {
+    private static final int WHITE = 0;
+    private static final int GRAY = 1;
+    private static final int BLACK = 2;
+
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        ArrayList<Integer>[] adj = new ArrayList[numCourses];
+        for (int i = 0; i < numCourses; i++) {
+            adj[i] = new ArrayList<>();
+        }
+        for (int[] pre : prerequisites) {
+            adj[pre[1]].add(pre[0]);
+        }
+        int[] colors = new int[numCourses];
+        for (int i = 0; i < numCourses; i++) {
+            if (colors[i] == WHITE && !adj[i].isEmpty() && hasCycle(adj, i, colors)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private boolean hasCycle(ArrayList<Integer>[] adj, int node, int[] colors) {
+        colors[node] = GRAY;
+        for (int nei : adj[node]) {
+            if (colors[nei] == GRAY) {
+                return true;
+            }
+            if (colors[nei] == WHITE && hasCycle(adj, nei, colors)) {
+                return true;
+            }
+        }
+        colors[node] = BLACK;
+        return false;
+    }
+}
diff --git a/src/main/java/g0201_0300/s0208_implement_trie_prefix_tree/Trie.java b/src/main/java/g0201_0300/s0208_implement_trie_prefix_tree/Trie.java
new file mode 100644
index 000000000..5d1e18404
--- /dev/null
+++ b/src/main/java/g0201_0300/s0208_implement_trie_prefix_tree/Trie.java
@@ -0,0 +1,64 @@
+package g0201_0300.s0208_implement_trie_prefix_tree;
+
+@SuppressWarnings("java:S1104")
+public class Trie {
+    static class TrieNode {
+        // Initialize your data structure here.
+        public TrieNode[] children;
+        public boolean isWord;
+
+        public TrieNode() {
+            children = new TrieNode[26];
+        }
+    }
+
+    private TrieNode root;
+    private boolean startWith;
+
+    public Trie() {
+        root = new TrieNode();
+    }
+
+    // Inserts a word into the trie.
+    public void insert(String word) {
+        insert(word, root, 0);
+    }
+
+    private void insert(String word, TrieNode root, int idx) {
+        if (idx == word.length()) {
+            root.isWord = true;
+            return;
+        }
+        int index = word.charAt(idx) - 'a';
+        if (root.children[index] == null) {
+            root.children[index] = new TrieNode();
+        }
+        insert(word, root.children[index], idx + 1);
+    }
+
+    // Returns if the word is in the trie.
+    public boolean search(String word) {
+        return search(word, root, 0);
+    }
+
+    public boolean search(String word, TrieNode root, int idx) {
+        if (idx == word.length()) {
+            startWith = true;
+            return root.isWord;
+        }
+        int index = word.charAt(idx) - 'a';
+        if (root.children[index] == null) {
+            startWith = false;
+            return false;
+        }
+
+        return search(word, root.children[index], idx + 1);
+    }
+
+    // Returns if there is any word in the trie
+    // that starts with the given prefix.
+    public boolean startsWith(String prefix) {
+        search(prefix);
+        return startWith;
+    }
+}
diff --git a/src/main/java/g0201_0300/s0209_minimum_size_subarray_sum/Solution.java b/src/main/java/g0201_0300/s0209_minimum_size_subarray_sum/Solution.java
new file mode 100644
index 000000000..49aa30392
--- /dev/null
+++ b/src/main/java/g0201_0300/s0209_minimum_size_subarray_sum/Solution.java
@@ -0,0 +1,33 @@
+package g0201_0300.s0209_minimum_size_subarray_sum;
+
+public class Solution {
+    public int minSubArrayLen(int target, int[] nums) {
+        int i = 0;
+        int j = 0;
+        int sum = 0;
+        int min = Integer.MAX_VALUE;
+        while (j < nums.length) {
+            sum += nums[j];
+            if (sum < target) {
+                j++;
+            } else {
+                while (i <= j) {
+                    if (sum - nums[i] >= target) {
+                        sum = sum - nums[i];
+                        i++;
+                    } else {
+                        break;
+                    }
+                }
+                if (j - i + 1 < min) {
+                    min = j - i + 1;
+                }
+                j++;
+            }
+        }
+        if (min == Integer.MAX_VALUE) {
+            return 0;
+        }
+        return min;
+    }
+}
diff --git a/src/test/java/g0201_0300/s0207_course_schedule/SolutionTest.java b/src/test/java/g0201_0300/s0207_course_schedule/SolutionTest.java
new file mode 100644
index 000000000..9d4f62e8f
--- /dev/null
+++ b/src/test/java/g0201_0300/s0207_course_schedule/SolutionTest.java
@@ -0,0 +1,18 @@
+package g0201_0300.s0207_course_schedule;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void canFinish() {
+        assertThat(new Solution().canFinish(2, new int[][] {{1, 0}}), equalTo(true));
+    }
+
+    @Test
+    public void canFinish2() {
+        assertThat(new Solution().canFinish(2, new int[][] {{1, 0}, {0, 1}}), equalTo(false));
+    }
+}
diff --git a/src/test/java/g0201_0300/s0208_implement_trie_prefix_tree/TrieTest.java b/src/test/java/g0201_0300/s0208_implement_trie_prefix_tree/TrieTest.java
new file mode 100644
index 000000000..d8ab47973
--- /dev/null
+++ b/src/test/java/g0201_0300/s0208_implement_trie_prefix_tree/TrieTest.java
@@ -0,0 +1,23 @@
+package g0201_0300.s0208_implement_trie_prefix_tree;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class TrieTest {
+    @Test
+    public void trie() {
+        Trie trie = new Trie();
+        trie.insert("apple");
+        // return True
+        assertThat(trie.search("apple"), equalTo(true));
+        // return False
+        assertThat(trie.search("app"), equalTo(false));
+        // return True
+        assertThat(trie.startsWith("app"), equalTo(true));
+        trie.insert("app");
+        // return True
+        assertThat(trie.search("app"), equalTo(true));
+    }
+}
diff --git a/src/test/java/g0201_0300/s0209_minimum_size_subarray_sum/SolutionTest.java b/src/test/java/g0201_0300/s0209_minimum_size_subarray_sum/SolutionTest.java
new file mode 100644
index 000000000..d09927de7
--- /dev/null
+++ b/src/test/java/g0201_0300/s0209_minimum_size_subarray_sum/SolutionTest.java
@@ -0,0 +1,24 @@
+package g0201_0300.s0209_minimum_size_subarray_sum;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void minSubArrayLen() {
+        assertThat(new Solution().minSubArrayLen(7, new int[] {2, 3, 1, 2, 4, 3}), equalTo(2));
+    }
+
+    @Test
+    public void minSubArrayLen2() {
+        assertThat(new Solution().minSubArrayLen(4, new int[] {1, 4, 4}), equalTo(1));
+    }
+
+    @Test
+    public void minSubArrayLen3() {
+        assertThat(
+                new Solution().minSubArrayLen(11, new int[] {1, 1, 1, 1, 1, 1, 1, 1}), equalTo(0));
+    }
+}