前序遍历就可以了
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
public class Solution {
    int length = 0;
    int maxlength = 0;
    public int TreeDepth(TreeNode root) {
        if(root == null){
            return length;
        }
        ++length;
        maxlength = maxlength>length?maxlength:length;
        TreeDepth(root.left);
        TreeDepth(root.right);
        --length;
        return maxlength;
    }
}