返回最大的密钥严格小于C++ Map中给定的密钥

#include<iostream>
#include <map>

int main() {
    std::map<int, std::string> my_map;
    my_map[1] = "one";
    my_map[3] = "three";
    my_map[5] = "five";
    my_map[7] = "seven";

    int key = 4;
    auto it = my_map.upper_bound(key);

    if (it != my_map.begin()) {
        --it;
        std::cout << "The largest key strictly less than "<< key << " is " << it->first
                  << " with value " << it->second<< std::endl;
    } else {
        std::cout << "No key strictly less than "<< key << " was found."<< std::endl;
    }

    return 0;
}