LRU Cache [LeetCode]

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

Solution: 

class LRUCache{
public:
    vector<int> keys;
    unordered_map<int, int> map;
    int size = 0;
    LRUCache(int capacity) {
        size = capacity;
    }
    
    void adjust(int key){
        int idx = -1;
        for(int i = keys.size() - 1; i >= 0 ; i --)
            if(keys[i] == key){
                idx = i;
                break;
            }
        
        if(idx == -1)
            return;
            
        keys.erase(keys.begin() + idx);
        keys.push_back(key);
    }
    
    int get(int key) {
        if(map.find(key) == map.end()){
            return -1;
        }else{
            adjust(key);
            return map[key];
        }
    }
    
    void set(int key, int value) {
        if(map.find(key) != map.end()){
            map[key] = value;
            adjust(key);
            return;
        }
        
        if(keys.size() >= size ){ 
            int key_to_erase = keys[0];
            keys.erase(keys.begin());
            map.erase(key_to_erase);
        }
    
        keys.push_back(key);
        map[key] = value;
    }
};