insert-delete-getrandom-o1

// 参考了下面一些讨论的解法
// https://discuss.leetcode.com/topic/53235/java-with-hashtable-arraylist/2

class RandomizedSet {
    vector<int> vec;
    unordered_map<int, int> umap;
public:
    /** 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 (umap.find(val) != umap.end()) {
            return false;
        }
        umap[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 (umap.find(val) == umap.end()) {
            return false;
        }
        int pos = umap[val];
        int back = vec.back();
        umap[back] = pos;
        vec[pos] = back;
        vec.erase(vec.begin()+vec.size()-1);
        umap.erase(val);
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        int randpos = rand() % vec.size();
        return vec[randpos];
    }
};

/**
 * 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();
 */