二叉树的后序遍历

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整型一维数组
     */
    public int[] postorderTraversal (TreeNode root) {
	  //思路与上一题类似
        // write code here
        if(root==null)
        return new int[0];
        List<Integer> list = new ArrayList<>();
        postorder(root,list);
        int[] array = list.stream().mapToInt(Integer::intValue).toArray();
        return array;
    }

    public void postorder(TreeNode root, List<Integer> list){
        if(root==null)
        return;
        postorder(root.left,list);
        postorder(root.right,list);
        list.add(root.val);
    }
}