二十分钟精通二分搜索与二分答案~
二分模板:https://leetcode-cn.com/problems/find-first-and-last-position-of-element-in-sorted-array/
例题: http://poj.org/problem?id=1064 , https://www.luogu.com.cn/problem/P1396
习题: https://leetcode-cn.com/problems/xiao-zhang-shua-ti-ji-hua