递归

• 当前节点深度为左右孩子的最大深度 + 1

  public class Solution {
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
  
        return Math.max(TreeDepth(root.left), TreeDepth(root.right)) + 1;
    }
  }

非递归

• 层次遍历树，每一层+1

  import java.util.*;
  public class Solution {
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
  
        Queue<TreeNode> queue = new LinkedList();
        int depth = 0;
        queue.offer(root);
  
        // 层次遍历
        while(!queue.isEmpty()) {
            depth++;
            int length = queue.size();
            // 出该层节点，且压入下一层
            while(length-- > 0) {
                TreeNode node = queue.poll();
                if(node.left != null) {
                    queue.offer(node.left);
                }
                if(node.right != null) {
                    queue.offer(node.right);
                }
            }
        }
  
        return depth;
    }
  }