379 Design Phone Directory

Design a Phone Directory which supports the following operations:
1. 1.
`get`
: Provide a number which is not assigned to anyone.
2. 2.
`check`
: Check if a number is available or not.
3. 3.
`release`
: Recycle or release a number.
Example:
// Init a phone directory containing a total of 3 numbers: 0, 1, and 2.
PhoneDirectory directory = new PhoneDirectory(3);
// It can return any available phone number. Here we assume it returns 0.
directory.get();
// Assume it returns 1.
directory.get();
// The number 2 is available, so return true.
directory.check(2);
// It returns 2, the only number that is left.
directory.get();
// The number 2 is no longer available, so return false.
directory.check(2);
// Release number 2 back to the pool.
directory.release(2);
// Number 2 is available again, return true.
directory.check(2);
The Idea: Use a 2 hash tables for reserved and free numbers. Append and erase between these two hash tables are as elements get reserved and freed.
Complexity: O(1) time, O(N) space
class PhoneDirectory {
public:
/** Initialize your data structure here
@param maxNumbers - The maximum numbers that can be stored in the phone directory. */
PhoneDirectory(int maxNumbers) {
free.reserve(maxNumbers);
reserved.reserve(maxNumbers);
for (int i = 0; i < maxNumbers; i++) {
free.insert(i);
}
}
/** Provide a number which is not assigned to anyone.
@return - Return an available number. Return -1 if none is available. */
int get() {
if (free.empty()) return -1;
int get = *(free.begin());
free.erase(get);
reserved.insert(get);
return get;
}
/** Check if a number is available or not. */
bool check(int number) {
return free.find(number) != free.end();
}
/** Recycle or release a number. */
void release(int number) {
reserved.erase(number);
free.insert(number);
}
unordered_set<int> free, reserved;
};
/**
* Your PhoneDirectory object will be instantiated and called as such:
* PhoneDirectory obj = new PhoneDirectory(maxNumbers);
* int param_1 = obj.get();
* bool param_2 = obj.check(number);
* obj.release(number);
*/