binary-tree-postorder-traversa二叉树的后序遍历

代码：

package com.niuke.p6;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    //二叉树的后序遍历
    public ArrayList<Integer> postorderTraversal (TreeNode root) {
        // write code here
        ArrayList<Integer> list = new ArrayList<>();
        if(root == null)
            return list;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode pre = null;
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode cur = stack.peek();
            if((cur.left == null && cur.right == null)||(pre != null && (pre == cur.left || pre == cur.right))) {
                list.add(cur.val);
                stack.pop();
                pre = cur;
            }else {
                if(cur.right != null) {
                    stack.push(cur.right);
                }
                if(cur.left != null) {
                    stack.push(cur.left);
                }
            }
        }
        
        
        return list;
    }

}