二进制搜索std：map

Question

Using c++, if I have n integers in a std::map , is it possible to efficiently search the largest element that is smaller than k in std::map ?

For example I have {1, 3, 5, 6} and k is 4 the returned value should be 3

I know std::map can search in log(n) , if the it is exact match.

Answer 1

使用lower_bound并将返回的迭代器减一。

Answer 2

You can use method lower_bound . For example

#include <iostream>
#include <map>

int main() 
{
    std::map<int, int> m = { { 1, 0 }, { 3, 0 }, { 5, 0 }, { 6, 0 } };

    auto it = m.lower_bound( 4 );

    if ( it != m.begin() )
    {
        std::cout << ( --it )->first << std::endl;
    }

    return 0;
}

Answer 3

Look into std::map::lower_bound() . Note that it will return an exact match if there is one. std::set has this as well.