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// @Title: 二叉树的后序遍历 (Binary Tree Postorder Traversal)
// @Author: 15816537946@163.com
// @Date: 2020-06-25 07:15:36
// @Runtime: 0 ms
// @Memory: 2.1 MB
/*
 * @lc app=leetcode.cn id=145 lang=golang
 *
 * [145] 二叉树的后序遍历
 *
 * https://leetcode-cn.com/problems/binary-tree-postorder-traversal/description/
 *
 * algorithms
 * Hard (67.10%)
 * Likes:    146
 * Dislikes: 0
 * Total Accepted:    25.5K
 * Total Submissions: 38K
 * Testcase Example:  '[1,null,2,3]'
 *
 * 给定一个二叉树,返回它的 后序 遍历。
 * 
 * 示例:
 * 
 * 输入: [1,null,2,3]  
 * ⁠  1
 * ⁠   \
 * ⁠    2
 * ⁠   /
 * ⁠  3 
 * 
 * 输出: [3,2,1]
 * 
 * 进阶: 递归算法很简单,你可以通过迭代算法完成吗?
 * 
 */
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
 func postorderTraversal(root *TreeNode) []int {
	if root == nil {
		return nil
	}
	res := make([]int, 0)
	nodes := make([]*TreeNode, 1, 1024)
	nodes[0] = root
	for len(nodes) > 0 {
		node := nodes[len(nodes)-1]
		nodes = nodes[:len(nodes)-1]
		res = append(res, node.Val)
		if node.Left != nil {
			nodes = append(nodes, node.Left)
		}
		if node.Right != nil {
			nodes = append(nodes, node.Right)
		}
	}

	reverse(res)
	return res
}

func reverse(arr []int) {
	lo, hi :=0, len(arr)-1
	for lo < hi {
		arr[lo],arr[hi] = arr[hi], arr[lo]
		lo++
		hi--
	}
}