如何在C++中对向量对的第二个元素执行lower_bound操作？

#include <algorithm>
#include <vector>

bool compare(std::pair<int, int> a, std::pair<int, int> b) {
    return a.second < b.second;
}

int main() {
    std::vector<std::pair<int, int>> vec = {{1, 5}, {2, 3}, {3, 2}, {4, 7}, {5, 4}};
    std::sort(vec.begin(), vec.end(), compare);
    
    int target = 4;
    auto it = std::lower_bound(vec.begin(), vec.end(), target, compare);
    
    if (it != vec.end()) {
        // 找到了第一个大于等于给定值的元素
        int index = std::distance(vec.begin(), it);
        std::cout << "Found at index: " << index << std::endl;
        std::cout << "Element: (" << vec[index].first << ", " << vec[index].second << ")" << std::endl;
    } else {
        // 没有找到大于等于给定值的元素
        std::cout << "Not found!" << std::endl;
    }
    
    return 0;
}