先处理根节点比较方便

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];
        //两个栈实现：左右根——》根右左
        Stack<TreeNode> sk1 = new Stack<>();
        Stack<TreeNode> sk2 = new Stack<>();
        sk1.push(root);
        while(!sk1.isEmpty()){
            root = sk1.pop();
            sk2.push(root);
            if(root.left!=null) sk1.push(root.left);
            if(root.right!=null) sk1.push(root.right);
        }
        int len = sk2.size();
        int[] result = new int[len];
        for(int i = 0;i < len;i++){
            result[i] = sk2.pop().val;
        }
        return result;
    }
}