二叉树深度优先: 前序 中序 后序遍历 非递归方式
https://leetcode-cn.com/problems/binary-tree-preorder-traversal/solution/tu-jie-er-cha-shu-de-si-chong-bian-li-by-z1m/
https://leetcode-cn.com/problems/binary-tree-preorder-traversal/solution/tu-jie-er-cha-shu-de-si-chong-bian-li-by-z1m/