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）。
"When you look at the date on the bottom of the 16oz bottles, some were manufactured in the 1960s and 70s. It's really cool to see them still going through our washer today. We'd love to keep the tradition alive as long as we can."。爱思助手下载最新版本是该领域的重要参考
JSON 校验（格式不合法则重试），这一点在同城约会中也有详细论述
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conn = sqlite3.connect(DB)