104. Maximum Depth of Binary Tree.java

E
tags: DFS, Tree

给一个binary tree, 找最深depth

#### DFS
- 这里要走过所有的node, 所以dfs非常合适
- Divide and conquer. 
- 维持一个最大值: Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
- 注意check root == null

#### Note
- BFS is doable as well, but a bit more code to write: tracks largest level we reach

```
/*
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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
return its depth = 3.

*/

/*
Thinking process:
check if root is null, return 0 if so.
Divide and return integer as the depth
Conquer: find the max and return depth + 1.
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}

```