add 104

Author: andavid

README.md

@@ -52,6 +52,13 @@
 | 234  | [Palindrome Linked List][234]               |
 | 141  | [Linked List Cycle][141]                    |
 
+**Trees**
+
+| #    | Title                                       |
+| :--: | :------------------------------------------ |
+| 104  | [Maximum Depth of Binary Tree][104]         |
+
+
 **其他**
 
 | #    | Title                                       |
@@ -91,3 +98,4 @@
 [021]: https://github.com/andavid/leetcode-java/blob/master/note/021/README.md
 [234]: https://github.com/andavid/leetcode-java/blob/master/note/234/README.md
 [141]: https://github.com/andavid/leetcode-java/blob/master/note/141/README.md
+[104]: https://github.com/andavid/leetcode-java/blob/master/note/104/README.md

note/104/README.md

@@ -0,0 +1,49 @@
+# [Maximum Depth of Binary Tree][title]
+
+## Description
+
+Given a binary tree, find its maximum depth.
+
+The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
+
+For example:
+Given binary tree [3,9,20,null,null,15,7],
+
+```
+    3
+   / \
+  9  20
+    /  \
+   15   7
+```
+
+return its depth = 3.
+
+## 思路
+
+递归。求二叉树 root 节点左子树和右子树的深度，其中较大的深度加 1 即为二叉树的深度。
+
+## [完整代码][src]
+
+```java
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+  public int maxDepth(TreeNode root) {
+    if (root == null) {
+      return 0;
+    }
+    return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/maximum-depth-of-binary-tree
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_104/Solution.java

src/com/andavid/leetcode/_104/Solution.java

@@ -0,0 +1,36 @@
+class Solution {
+  public static int maxDepth(TreeNode root) {
+    if (root == null) {
+      return 0;
+    }
+    return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
+  }
+
+  public static class TreeNode {
+    int val;
+    TreeNode left;
+    TreeNode right;
+    TreeNode(int x) {
+      val = x;
+    }
+  }
+
+  public static void main(String[] args) {
+    TreeNode root = new TreeNode(3);
+    TreeNode node1 = new TreeNode(9);
+    TreeNode node2 = new TreeNode(20);
+    TreeNode node3 = new TreeNode(15);
+    TreeNode node4 = new TreeNode(7);
+
+    root.left = node1;
+    root.right = node2;
+    node1.left = node1.right = null;
+    node2.left = node3;
+    node2.right = node4;
+    node3.left = node3.right = null;
+    node4.left = node4.right = null;
+
+    System.out.println(maxDepth(root));
+  }
+
+}