diff --git a/Week_03/id_9/LeetCode_104_9.java b/Week_03/id_9/LeetCode_104_9.java
new file mode 100644
index 00000000..848cacd9
--- /dev/null
+++ b/Week_03/id_9/LeetCode_104_9.java
@@ -0,0 +1,39 @@
+package com.github.lifelab.leetcode.problemset;
+
+/**
+ * 二叉树的最大深度 @https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/
+ *
+ * @author Weichao Li (liweichao0102@gmail.com)
+ * @since 2019-06-09
+ */
+public class Solution104 {
+
+    public int maxDepth(TreeNode root) {
+        return depth(root, 0);
+    }
+
+    private int depth(TreeNode node, int depth) {
+
+        if (node == null) {
+            return 0;
+        }
+
+        depth++;
+
+        if (node.left == null && node.right == null) {
+            return depth;
+        }
+
+        int l = 0, r = 0;
+
+        if (node.left != null) {
+            l = depth(node.left, depth);
+        }
+
+        if (node.right != null) {
+            r = depth(node.right, depth);
+        }
+
+        return Math.max(l, r);
+    }
+}
diff --git a/Week_03/id_9/LeetCode_111_9.java b/Week_03/id_9/LeetCode_111_9.java
new file mode 100644
index 00000000..a63dea9b
--- /dev/null
+++ b/Week_03/id_9/LeetCode_111_9.java
@@ -0,0 +1,40 @@
+package com.github.lifelab.leetcode.problemset;
+
+import java.util.Objects;
+
+/**
+ * 二叉树的最小深度 @see https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/submissions/
+ *
+ * @author Weichao Li (liweichao0102@gmail.com)
+ * @since 2019-07-07
+ */
+public class Solution111 {
+
+    public int minDepth(TreeNode root) {
+        return minDepth(root, 0);
+    }
+
+    public int minDepth(TreeNode node, int currentDepth) {
+
+        if (node == null) {
+            return currentDepth;
+        }
+
+        currentDepth++;
+
+        if (Objects.isNull(node.left) && Objects.isNull(node.right)) {
+            return currentDepth;
+        } else {
+
+            int minDepth = Integer.MAX_VALUE;
+            if (node.left != null) {
+                minDepth = Math.min(minDepth(node.left, currentDepth), minDepth);
+            }
+            if (node.right != null) {
+                minDepth = Math.min(minDepth(node.right, currentDepth), minDepth);
+            }
+            return minDepth;
+        }
+
+    }
+}
diff --git a/Week_03/id_9/LeetCode_547_9.java b/Week_03/id_9/LeetCode_547_9.java
new file mode 100644
index 00000000..71182461
--- /dev/null
+++ b/Week_03/id_9/LeetCode_547_9.java
@@ -0,0 +1,50 @@
+package com.github.lifelab.leetcode.problemset;
+
+/**
+ * 朋友圈 @https://leetcode-cn.com/problems/friend-circles/
+ *
+ * @author Weichao Li (liweichao0102@gmail.com)
+ * @since 2019-07-07
+ */
+public class Solution547 {
+
+
+    public int findCircleNum(int[][] M) {
+        //二维数组长度，即所有人的个数
+        int length = M.length;
+        //统计朋友圈个数
+        int count = 0;
+        //访问标志
+        boolean[] flag = new boolean[length];
+        //对于每个人
+        for (int i = 0; i < length; i++) {
+            //如果未被访问
+            if (!flag[i]) {
+                //深度优先搜索，访问
+                dfs(i, M, flag);
+                //朋友圈个数+1
+                count++;
+            }
+        }
+        return count;
+    }
+
+    /**
+     * 深度优先搜索
+     *
+     * @param i    人的位置
+     * @param M    朋友圈
+     * @param flag 访问标识
+     */
+    public void dfs(int i, int[][] M, boolean[] flag) {
+        //当前位占位
+        flag[i] = true;
+
+        for (int j = 0; j < M[i].length; j++) {
+
+            if (!flag[j] && M[i][j] == 1) {
+                dfs(j, M, flag);
+            }
+        }
+    }
+}