如何优化C++键值程序以获得更快的运行时间

这是一个可以编辑的程序，据我所知，代码是正确的，只是太慢了。老实说，我在这里迷路了，我知道我的for循环可能是让我这么慢的原因，也许使用迭代器？

// <algorithm>, <list>, <vector>
// YOU CAN CHANGE/EDIT ANY CODE IN THIS FILE AS LONG AS SEMANTICS IS UNCHANGED
#include <algorithm>
#include <list>
#include <vector>
class key_value_sequences {
private:
std::list<std::vector<int>> seq;
std::vector<std::vector<int>> keyref;
public:
// YOU SHOULD USE C++ CONTAINERS TO AVOID RAW POINTERS
// IF YOU DECIDE TO USE POINTERS, MAKE SURE THAT YOU MANAGE MEMORY PROPERLY
// IMPLEMENT ME: SHOULD RETURN SIZE OF A SEQUENCE FOR GIVEN KEY
// IF NO SEQUENCE EXISTS FOR A GIVEN KEY RETURN 0
int size(int key) const;
// IMPLEMENT ME: SHOULD RETURN POINTER TO A SEQUENCE FOR GIVEN KEY
// IF NO SEQUENCE EXISTS FOR A GIVEN KEY RETURN nullptr
const int* data(int key) const;
// IMPLEMENT ME: INSERT VALUE INTO A SEQUENCE IDENTIFIED BY GIVEN KEY
void insert(int key, int value);
}; // class key_value_sequences

int key_value_sequences::size(int key) const {
//checks if the key is invalid or the count vector is empty.
if(key<0 || keyref[key].empty()) return 0;
// sub tract 1 because the first element is the key to access the count
return keyref[key].size() -1;
}
const int* key_value_sequences::data(int key) const {
//checks if key index or ref vector is invalid
if(key<0 || keyref.size() < static_cast<unsigned int>(key+1)) {
return nullptr;
}
// ->at(1) accesses the count (skipping the key) with a pointer
return &keyref[key].at(1);
}
void key_value_sequences::insert(int key, int value) {
//checks if key is valid and if the count vector needs to be resized
if(key>=0 && keyref.size() < static_cast<unsigned int>(key+1)) {
keyref.resize(key+1);
std::vector<int> val;
seq.push_back(val);
seq.back().push_back(key);
seq.back().push_back(value);
keyref[key] = seq.back();
}
//the index is already valid
else if(key >=0) keyref[key].push_back(value);
}
#endif // A2_HPP

这是a2.cpp，这只是测试a2.hpp的功能，此代码不能更改

// DO NOT EDIT THIS FILE !!!
// YOUR CODE MUST BE CONTAINED IN a2.hpp ONLY
#include <iostream>
#include "a2.hpp"

int main(int argc, char* argv[]) {
key_value_sequences A;
{
key_value_sequences T;
// k will be our key
for (int k = 0; k < 10; ++k) {  //the actual tests will have way more than 10 sequences.
// v is our value
// here we are creating 10 sequences:
// key = 0, sequence = (0)
// key = 1, sequence = (0 1)
// key = 2, sequence = (0 1 2)
// ...
// key = 9, sequence = (0 1 2 3 4 5 6 7 8 9)
for (int v = 0; v < k + 1; ++v) T.insert(k, v);
}
T = T;
key_value_sequences V = T;
A = V;
}
std::vector<int> ref;
if (A.size(-1) != 0) {
std::cout << "fail" << std::endl;
return -1;
}
for (int k = 0; k < 10; ++k) {
if (A.size(k) != k + 1) {
std::cout << "fail";
return -1;
} else {
ref.clear();
for (int v = 0; v < k + 1; ++v) ref.push_back(v);
if (!std::equal(ref.begin(), ref.end(), A.data(k))) {
std::cout << "fail 3 " << A.data(k) << " " << ref[k];
return -1;
}
}
}
std::cout << "pass" << std::endl;
return 0;
} // main

如果有人能帮助我提高代码效率，我将非常感激，谢谢。