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用stack，把比当前小的都pop出来，说明当前到了某个parent的右边keep 一个lowBound是当前最后一个弹出的值，若当前值小于lowBound则无效优化为空间O(1)的方法是把原先数组用作stack也可以用divide&conquer，查看后面是不是先都比root小，再都比root大，然后迭代

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