二分搜索模板

转载自公众号labuladong的力扣题解：code https://leetcode-cn.com/problems/find-first-and-last-position-of-element-in-sorted-array/solution/er-fen-cha-zhao-suan-fa-xi-jie-xiang-jie-by-labula/索引 二分查找场景：寻找一个数、寻找左侧边界、寻

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