std::map 查询某个key对应的值，请用c++实现

可以使用 `std::map` 中的 `find` 函数来查询某个 key 对应的值。示例代码如下：

#include <iostream>
#include <map>

int main() {
    std::map<std::string, int> myMap;
    myMap["apple"] = 10;
    myMap["banana"] = 20;
    myMap["cherry"] = 30;

    // 查询 key 为 "banana" 的值
    auto it = myMap.find("banana");
    if (it != myMap.end()) {
        std::cout << "The value of banana is: " << it->second << std::endl;
    } else {
        std::cout << "banana is not in the map." << std::endl;
    }

    // 查询不存在的 key
    it = myMap.find("durian");
    if (it != myMap.end()) {
        std::cout << "The value of durian is: " << it->second << std::endl;
    } else {
        std::cout << "durian is not in the map." << std::endl;
    }

    return 0;
}

输出：

The value of banana is: 20
durian is not in the map.