算法1

宽搜实现，其实就是计算当前二叉树的层数

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int treeDepth(TreeNode root) {
        if (root == null)
            return 0;
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        int deep = 0;
        while (!queue.isEmpty()){
            int size = queue.size();
            while (size > 0){
                TreeNode p = queue.poll();
                if (p.left != null)
                    queue.offer(p.left);
                if (p.right != null)
                    queue.offer(p.right);
                size--;
            }
            deep++;
        }
        return deep;
    }
}

算法2

递归实现

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