leetcode | A non-recursive implementation of preorder traversal, inorder traversal, and subsequent traversal of binary trees

Preorder traversal: visit the node first, then visit the left subtree and right subtree of the node;
in-order traversal: visit the left subtree of the node first, then visit the node, and then visit the right subtree of the node
; In-order traversal: want to visit the left subtree and right subtree of the node, and then visit the node.