描述：给定字符串 num（非负整数）和整数 k，移除 k 位数字，使剩余数字最小。以字符串形式返回（不含前导零）。
The writer has a simple interface: write(), writev() for batched writes, end() to signal completion, and abort() for errors. That's essentially it.
。safew官方下载对此有专业解读
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公式： f(x)=x⋅Φ(x)≈0.5x(1+tanh[2/π(x+0.044715x3)])