递归版本

class Solution {
    public int treeDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int left = treeDepth(root.left);
        int right = treeDepth(root.right);
        return Math.max(left, right) + 1;
    }
}

迭代版本（层次遍历的思想，计算层数即可）

class Solution {
    public int treeDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        Queue<TreeNode> nodes = new LinkedList<>();
        nodes.offer(root);
        int depth = 0;

        while (!nodes.isEmpty()) {
            int curSize = nodes.size();
            for (int i = 0 ; i < curSize; i++) {
                TreeNode curNode = nodes.poll();
                if (curNode.left != null) {
                    nodes.offer(curNode.left);
                }
                if (curNode.right != null) {
                    nodes.offer(curNode.right);
                }
            }
            depth++;
        }
        return depth;
    }
}