A quadtree does the same thing for two-dimensional space. It takes a rectangular region and divides it into four equal quadrants: northwest, northeast, southwest, southeast. If a quadrant has too many points in it, it subdivides again and again. Each subdivision creates smaller and smaller cells where points are densely packed.
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// 注意：slice(0, 负数)会返回空字符串，需兼容（比如stack长度<k时，slice后为空）
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