使用stl :: list插入和擦除的顺序

Question

When I am doing practice on leetcode, I met a problem like this:

I used a stl::list container as cache for LRU algorithm. But the sequence of erasing an item and inserting an item made the result different.

I know that it is actually a double list as stl::list . And the sequence of inserting and erasing should not matter when I use iterator.

The code is here

class LRUCache{
public:

map<int, list<pair<int,int>>::iterator> mKey;
list<pair<int,int>> lCache;
int cap;


LRUCache(int capacity) {
    cap = capacity;
}

int get(int key) {
    auto iter = mKey.find(key);
    if(iter != mKey.end()) {
        int value = (iter->second)->second;


        //**the sequence of next two lines can not be changed！***
        lCache.erase(iter->second);
        mKey[key] = lCache.insert(lCache.begin(), make_pair(key,value));

        return value;
    }
    return -1;
}

void set(int key, int value) {
    auto iter = mKey.find(key);
    if(iter == mKey.end()) {
        if(lCache.size() < cap) {
            mKey[key] = lCache.insert(lCache.begin(), make_pair(key,value));
        }
        else{
            mKey[key] = lCache.insert(lCache.begin(), make_pair(key,value));
            mKey.erase(lCache.back().first);
            lCache.pop_back();
        }
    }
    else {
        lCache.erase(iter->second);
        mKey[key] = lCache.insert(lCache.begin(), make_pair(key,value));
    }

}
};

Answer 1

It's not quite clear what you are asking. If your question is why these two lines can't be reordered:

    //**the sequence of next two lines can not be changed！***
    lCache.erase(iter->second);
    mKey[key] = lCache.insert(lCache.begin(), make_pair(key,value));

then that's simple. iter points to the same node as mKey[key] , so the assignment actually changes the value of iter->second . If the assignment would happen first, then iter->second would point to the freshly inserted list node, not the previously existing one.