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 = ;
     LRUCache(int capacity) {
         size = capacity;
     }

     void adjust(int key){
         int idx = -;
         for(int i = keys.size() - ; i >=  ; i --)
             if(keys[i] == key){
                 idx = i;
                 break;
             }

         if(idx == -)
             return;

         keys.erase(keys.begin() + idx);
         keys.push_back(key);
     }

     int get(int key) {
         if(map.find(key) == map.end()){
             return -;
         }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[];
             keys.erase(keys.begin());
             map.erase(key_to_erase);
         }

         keys.push_back(key);
         map[key] = value;
     }
 };