LeetCode - Binary-Tree-Postorder-Traversal

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题目描述

Given a binary tree, return the postorder traversal of its nodes’ values.
For example:
Given binary tree{1,#,2,3},
1
\
2
/
3
return[3,2,1].

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.ArrayList;
import java.util.Stack;
public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> arrayList = new ArrayList<Integer>();
        Stack<TreeNode> stack1 = new Stack<TreeNode>();
        Stack<TreeNode> stack2 = new Stack<TreeNode>();

        if(root == null) return arrayList;

        stack1.add(root);
        while(!stack1.isEmpty()){
            TreeNode node = stack1.pop();
            stack2.push(node);
            if(node.left != null)   stack1.push(node.left);
            if(node.right != null)  stack1.push(node.right);
        }

        while(!stack2.isEmpty()){
            arrayList.add(stack2.pop().val);
        }
        return arrayList;
    }
}