lower_bound()和upper_bound()

lower_bound()和upper_bound() 是方便的在有序數組中二分查找的函數，並且在STL其他數據結構中也提供該方法（如map和set）。
但是兩函數並不是二分查找“小於”和“大於”的第一個元素。
lower_bound(first, last, val)大於等於val的第一個元素
upper_bound(first, last, val)嚴格大於val的第一個元素

lower_bound()

lower_bound - C++ Reference

Returns an iterator pointing to the first element in the range [first,last) which does not compare less than val.

not compare less than 不小於

upper_bound()

upper_bound - C++ Reference

Returns an iterator pointing to the first element in the range [first,last) which compares greater than val.

compares greater than 大於