Binary Tree Preorder Traversal

思路：
递归版太简单了。迭代版需要用到stack，由于preorder 是root-> left -> right。所以stack push的顺序是反的。先检查right 是否为空，push right, 再处理left。

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776. Split BST

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663. Equal Tree Partition

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532. K-diff Pairs in an Array

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