题解 | #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
     */
    private int j = 0;
    private void postOrder(ArrayList list, TreeNode b) {
        if (b != null) {
            postOrder(list, b.left);
            postOrder(list, b.right);
            list.add(b.val);
        }
    }
    public int[] postorderTraversal (TreeNode root) {
        // write code here
        ArrayList<Integer> list = new ArrayList();
        postOrder(list, root);
        int[] arr = new int[list.size()];
        list.forEach(i -> {
            arr[j++] = i;
        });
        return arr;
    }
}