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
     */
    public int[] postorderTraversal (TreeNode root) {
        // write code here
        if (root == null) {
            return new int[0];
        }
        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();
        stack1.push(root);
        TreeNode cur = root;
        while (!stack1.isEmpty()) {
            cur = stack1.pop();
            stack2.push(cur);
            if (cur.left != null) {
                stack1.push(cur.left);
            }
            if (cur.right != null) {
                stack1.push(cur.right);
            }
        }

        List<Integer> list = new ArrayList<>();
        while (!stack2.isEmpty()) {
            list.add(stack2.pop().val);
        }
        int[] res = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            res[i] = list.get(i);
        }
        return res;
    }






}