leetcode-380. 常数时间插入、删除和获取随机元素

思路：数组实现随机存取，哈希表定位元素的位置

class RandomizedSet {
public:

    vector<int> vec;
    default_random_engine e;
    uniform_int_distribution<int> u;
    unordered_map<int,int> memo;//key position
    /** Initialize your data structure here. */
    RandomizedSet() {

    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if(memo.find(val)!=memo.end())
         return false;
        memo[val]=vec.size();
        vec.push_back(val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(memo.find(val)==memo.end()) return false;
        int t=memo[val];
        vec[t]=vec[vec.size()-1];//用最后一个覆盖被删除的位置，然后移除最后一个元素
        memo[vec[t]]=t;//更新元素信息
        memo.erase(val);//索引删除
        vec.pop_back();//最后一个位置删除
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        return vec[u(e)%vec.size()];
    }
};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet* obj = new RandomizedSet();
 * bool param_1 = obj->insert(val);
 * bool param_2 = obj->remove(val);
 * int param_3 = obj->getRandom();
 */