The 80286 introduced "Protected Mode" in 1982. It was not popular. The mode was difficult to use, lacked paging, and offered no way to return to real mode without a hardware reset. The 80386, arriving three years later, made protection usable -- adding paging, a flat 32-bit address space, per-page User/Supervisor control, and Virtual 8086 mode so that DOS programs could run inside a protected multitasking system. These features made possible Windows 3.0, OS/2, and early Linux.
The best advice I can give to a person starting a business is to stay focused, don’t be discouraged by naysayers and don’t get overwhelmed by the process — you’re bigger than it and you can handle more than you think. Focus on small daily tasks, tackle them consistently, and over time, you’ll realize you’ve built something far bigger than you ever imagined. And that feeling is incredible.
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* 核心思路：倒序单调栈（适配你的学习习惯），时间复杂度O(n)，空间复杂度O(n)
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