Helium is used to pressurize tanks and help push fuel into the engines. Without proper helium flow, the rocket cannot safely fly. Because the upper stage is hard to reach at the launchpad, NASA rolled the stacked rocket back into the Vehicle Assembly Building, the sky-scraping hangar where it was originally put together.
Вячеслав Агапов
，这一点在Line官方版本下载中也有详细论述
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需要索引（如算距离、下标）：栈存索引，用 nums[stack.at(-1)] 取值比较。
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