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// @Title: 二叉树的中序遍历 (Binary Tree Inorder Traversal)
// @Author: 15816537946@163.com
// @Date: 2020-06-25 07:15:36
// @Runtime: 0 ms
// @Memory: 2 MB
/*
 * @lc app=leetcode.cn id=94 lang=golang
 *
 * [94] 二叉树的中序遍历
 *
 * https://leetcode-cn.com/problems/binary-tree-inorder-traversal/description/
 *
 * algorithms
 * Medium (67.87%)
 * Likes:    275
 * Dislikes: 0
 * Total Accepted:    58.2K
 * Total Submissions: 85.7K
 * Testcase Example:  '[1,null,2,3]'
 *
 * 给定一个二叉树,返回它的中序 遍历。
 *
 * 示例:
 *
 * 输入: [1,null,2,3]
 * ⁠  1
 * ⁠   \
 * ⁠    2
 * ⁠   /
 * ⁠  3
 *
 * 输出: [1,3,2]
 *
 * 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 inorderTraversal(root *TreeNode) []int {
	if root == nil {
		return nil
	}
	res := make([]int, 0)
	var travel func(*TreeNode)
	travel = func(node *TreeNode) {
		if node == nil {
			return
		}
		travel(node.Left)
		res = append(res, node.Val)
		travel(node.Right)
	}
	travel(root)
	return res
}
*/
func inorderTraversal(root *TreeNode) []int {
	if root == nil {
		return nil
	}
	nodes, res := []*TreeNode{}, []int{}
	for len(nodes) > 0 || root != nil {
		if root != nil {
			nodes = append(nodes, root)
			root = root.Left
		} else {
			node := nodes[len(nodes)-1]
			nodes = nodes[:len(nodes)-1]
			res = append(res, node.Val)
			root = node.Right
		}
	}

	return res
}