1. BFS层序遍历：
    import java.util.List;
import java.util.LinkedList;
public class Solution {
 public int TreeDepth(TreeNode root) {
     if(root == null) return 0;
     List<TreeNode> queue = new LinkedList<>();
     queue.add(root);
     List<TreeNode> tmp;
     int res = 0;
     while(!queue.isEmpty())
     {
         tmp = new LinkedList<>();
         for(TreeNode node : queue)
         {
             if(node.left != null) tmp.add(node.left);
             if(node.right != null) tmp.add(node.right);
         }
         queue = tmp;
         res++;
     }
     return res;
}
}
  2. DFS深度优先遍历（实际上是后续遍历）
    public class Solution {
 public int TreeDepth(TreeNode root) {
     if(root == null)
         return 0;
     int lval = TreeDepth(root.left);
     int rval = TreeDepth(root.right);
     return Math.max(lval , rval) + 1;
 }
}