Binary Tree Postorder Traversal

Problem

Given a binary tree, return the postorder traversal of its nodes' values. For example:

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Idea

使用stack实现，压栈时先压右子树再压左子树，只有找到叶子节点时才弹栈，压栈的时候注意把curNode与左右子树分开，这样才能保证从树底返回树顶。

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Solution

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