379. Design Phone Directory

I was thinking to build up a hash table for every number in the construction function. However, it turns out too costly. Actually, I only needs to have two arrays, with one storing numbers and another storing used tag. The idea behind is that we really don't have to care about the order of number that we dispensed and even who got the number. So don't think over too much. It's a very simple design.

1:  class PhoneDirectory {  
2:  private:  
3:    vector<int> numbers;  
4:    vector<bool> used;  
5:    int front;  
6:    int maxNumbers;  
7:  public:  
8:    /** Initialize your data structure here  
9:      @param maxNumbers - The maximum numbers that can be stored in the phone directory. */  
10:    PhoneDirectory(int maxNumbers) {  
11:      this->maxNumbers = maxNumbers;  
12:      numbers = vector<int>(maxNumbers, 0);  
13:      used = vector<bool>(maxNumbers, false);  
14:      front = 0;  
15:      for (int i = 0; i < maxNumbers; i++) {  
16:        numbers[i] = i;  
17:      }  
18:    }  
19:    /** Provide a number which is not assigned to anyone.  
20:      @return - Return an available number. Return -1 if none is available. */  
21:    int get() {  
22:      if (front == maxNumbers) return -1;  
23:      int ret = numbers[front++];  
24:      used[ret] = true;  
25:      return ret;  
26:    }  
27:    /** Check if a number is available or not. */  
28:    bool check(int number) {  
29:      if (number < 0 || number > maxNumbers-1) return false;  
30:      return !used[number];  
31:    }  
32:    /** Recycle or release a number. */  
33:    void release(int number) {  
34:      if (number >= 0 && number < maxNumbers && used[number]) {  
35:        numbers[--front] = number;  
36:        used[number] = false;  
37:      }  
38:    }  
39:  };  
40:  /**  
41:   * Your PhoneDirectory object will be instantiated and called as such:  
42:   * PhoneDirectory obj = new PhoneDirectory(maxNumbers);  
43:   * int param_1 = obj.get();  
44:   * bool param_2 = obj.check(number);  
45:   * obj.release(number);  
46:   */