Maximum Depth of Binary Tree

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.

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上一篇用DFS求了最小深度，这里用同样的方法求最大，所以只需在上一篇的基础上求出集合里面的最大值即可。Collections工具类非常好用，对list排序，求list中最大，最小值都ok。熟悉API还是很重要的。
代码如下：

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
private ArrayList<Integer> list = new ArrayList<Integer>();
public int maxDepth(TreeNode root) {
if(root==null) return 0;
dfs(root,0);
//Collections工具类非常好用，对list排序，求list中最大，最小值都ok
return  Collections.max(list);
}

public void dfs(TreeNode root,int count){
if(root==null){
return;
}else{
count++;
if(root.left==null&&root.right==null){
list.add(count);
return;
}
}

dfs(root.left,count);
dfs(root.right,count);

}

}