题解 | #统计农场牛数量#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    public int countNodes (TreeNode root) {
        // write code here
        List<Integer> list = new ArrayList<>();
        preOrder(root,list);
        return list.size();
    }

    public void preOrder(TreeNode node,List<Integer> list){
        if(node == null){
            return;
        }

        list.add(node.val);
        preOrder(node.left,list);
        preOrder(node.right,list);
    }
}