LeetCode #379. Design Phone Directory

题目描述：

Design a Phone Directory which supports the following operations:

1. get: Provide a number which is not assigned to anyone.
2. check: Check if a number is available or not.
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);

class PhoneDirectory {
public:
    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    PhoneDirectory(int maxNumbers) {
        for(int i=0;i<maxNumbers;i++) q.push(i);
    }
    
    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    int get() {
        if(q.empty()) return -1;
        int x=q.front();
        q.pop();
        used.insert(x);
        return x;
    }
    
    /** Check if a number is available or not. */
    bool check(int number) {
        return used.count(number)==0;
    }
    
    /** Recycle or release a number. */
    void release(int number) {
        if(used.count(number)==0) return;
        used.erase(number);
        q.push(number);
    }
    
private:
    queue<int> q;
    unordered_set<int> used;
};