一. 思路

先判根节点是否为空，为空则直接返回深度为0。用层次遍历方法，每遍历一层，深度就加1。

二. 代码

import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public int TreeDepth(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        int level = 0;

        if (root == null) return 0;
        queue.offer(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();

                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            level++;
        }
        return level;
    }
}