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# 379 Design Phone Directory

## 379. [Design Phone Directory](https://leetcode.com/problems/design-phone-directory/description/)

## 1. Question

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);
```

## 2. Implementation

```java
class PhoneDirectory {
    Set<Integer> usedNumbers;
    Queue<Integer> phoneNumbers;
    int max;

    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        max = maxNumbers;
        usedNumbers = new HashSet<>();
        phoneNumbers = new LinkedList<>();

        for (int i = 0; i < max; i++) {
            phoneNumbers.add(i);
        }
    }

    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    public int get() {
        if (phoneNumbers.size() <= 0) {
            return -1;
        }
        int number = phoneNumbers.remove();
        usedNumbers.add(number);
        return number;
    }

    /** Check if a number is available or not. */
    public boolean check(int number) {
        if (number >= max || number < 0) {
            return false;
        }
        return !usedNumbers.contains(number);
    }

    /** Recycle or release a number. */
    public void release(int number) {
        if (usedNumbers.remove(number)) {
            phoneNumbers.add(number);
        }
    }
}

/**
 * Your PhoneDirectory object will be instantiated and called as such:
 * PhoneDirectory obj = new PhoneDirectory(maxNumbers);
 * int param_1 = obj.get();
 * boolean param_2 = obj.check(number);
 * obj.release(number);
 */
```

## 3. Time & Space Complexity

时间复杂度get: O(1), check: O(1), release: O(1), 空间复杂度O(n)


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