方向遍历顺序说明下一个倒序（从右往左）先处理右侧，栈里存「右侧候选」上一个正序（从左往右）先处理左侧，栈里存「左侧候选」
Why did they plan to flood farmland?
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There is no one single plan for how to abandon ship, since everything will depend on, well, everything. But something has to happen, because a giant piece of space trash will be headed toward Earth—albeit very, very slowly, and with the oversight of the world’s top space engineers and scientists. But still, a worrisome situation. Ideally, the US Deorbit Vehicle, our Dragon, will be ready to shepherd the ISS to the atmosphere, over a safe zone in the Pacific Ocean.
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