Hi all,

The idea is to keep a vector for picking a random number and hash map (unordered_map in c++) for quick insert / removal. The map keeps the indices in the vector as the value so the corresponding entry in vector can be adjusted in O(1) time.

```
class RandomizedSet {
private:
unordered_map<int, int> indices; // the value is the index of the key
vector<int> vals;
public:
/** Initialize your data structure here. */
RandomizedSet() {
srand((int)time(0));
}
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
unordered_map<int, int>::iterator it = indices.find(val);
if (it == indices.end()) {
// add the new value's index
indices.insert(pair<int, int>(val, vals.size()));
vals.push_back(val);
return true;
}
return false;
}
/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
unordered_map<int, int>::iterator it = indices.find(val);
if (it == indices.end()) {
return false;
}
// remove val
int index = it->second;
vals[index] = vals[vals.size() - 1];
indices.find(vals[index])->second = index;
vals.pop_back();
indices.erase(it);
return true;
}
/** Get a random element from the set. */
int getRandom() {
int pick = rand() % vals.size();
return vals[pick];
}
};
```