1960s-1970s ATMs so primitive was the lack of computer systems that were
be sent there automatically... and then when you needed a little walking around
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思路：单调递增栈 + k 控制删除次数。高位越小整体越小，遇更小数字时弹出栈顶大数（仅当 k0）；栈空且当前为 0 则跳过（避免前导零）；若遍历完 k 仍0，从末尾再删 k 位。
For full control over the pipeline: