class Solution {
    public int treeDepth(TreeNode root) {
        if(root == null){
            return 0;
        }
        LinkedList<TreeNode> list = new LinkedList();
        list.add(root);
        int count = 0;
        while(root != null && !list.isEmpty()){
            int len = list.size();
            count++;
            while(len > 0){
                TreeNode tmp = list.pop();
                if(tmp.left != null) {
                    list.add(tmp.left);
                    root = tmp.left;
                }
                if(tmp.right != null){
                    list.add(tmp.right);
                    root = tmp.right;
                }
                len--;
            }
        } 
        return count;
    }

}