tips

https://cspiration.com/leetcodeClassificationcspiration.com

https://leetcode.com/problems/binary-tree-postorder-traversal/discuss/45551/Preorder-Inorder-and-Postorder-Iteratively-Summarization

注意: 102, 127 example

qSize 一定要提出來, 不然會錯(why...?

        int qSize = queue.size();
        for (int q = 0; q < qSize; q++) {