Given a binary tree, return the inorder traversal of its nodes' values.

For example:

Given binary tree [1,null,2,3],

1    \     2    /   3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

题意：不使用递归的方式，中序遍历二叉树

1. /**
2. * Definition for a binary tree node.
3. * function TreeNode(val) {
4. * this.val = val;
5. * this.left = this.right = null;
6. * }
7. */
8. /**
9. * @param {TreeNode} root
10. * @return {number[]}
11. */
12. let inorderTraversal = (root) => {
13. let res = [];
14. if (root == null) return res;
15. let stack = [];
16. var node = root;
17. while (node != null || stack.length != 0) {
18. while (node != null) {
19. stack.push(node);
20. node = node.left;
21. }
22. if (stack.length != 0) {
23. node = stack.pop();
24. res.push(node.val)
25. node = node.right;
26. }
27. }
28. return res;
29. }