379. Design Phone Directory

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);

S1: hashset & rand

存储已有的电话号码，每次随机生成，当使用过的号码很多时随机事件会很长

class PhoneDirectory {
public:
    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    PhoneDirectory(int maxNumbers) {
        n = maxNumbers;
    }

    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    int get() {
        if (used.size() == n) return -1;
        int num = rand() % n;
        while (used.find(num) != used.end()) {
            num = rand() % n;
        }
        used.insert(num);
        return num;
    }

    /** Check if a number is available or not. */
    bool check(int number) {
        if (number < 0 || number >= n || used.find(number) != used.end()) return false;
        return true;
    }

    /** Recycle or release a number. */
    void release(int number) {
        if (used.find(number) != used.end()) used.erase(number);
    }
private:
    int n;
    unordered_set<int> used;
};

S2：两个数组，顺序取号码

两个数组，一个存储可用的电话号码，顺序取出。一个存储每个号码对应在第一个数组中的index，除了初始函数外都是O(1)的操作

class PhoneDirectory {
public:
    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    PhoneDirectory(int maxNumbers) {
        idx = 0;
        nums.resize(maxNumbers);
        numToIdx = vector<int>(maxNumbers, -1);
        for (int i = 0; i < maxNumbers; ++i) nums[i] = i;
    }

    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    int get() {
        if (idx == nums.size()) return -1;
        numToIdx[nums[idx]] = idx;
        return nums[idx++];
    }

    /** Check if a number is available or not. */
    bool check(int number) {
        if (number >= 0 && number < nums.size() && numToIdx[number] == -1) return true;
        return false;
    }

    /** Recycle or release a number. */
    void release(int number) {
        if (number < 0 || number >= nums.size() || numToIdx[number] == -1) return;
        int i = numToIdx[number];
        idx--;  
        numToIdx[nums[idx]] = i;
        numToIdx[number] = -1;
        swap(nums[i], nums[idx]);       
    }
private:
    int idx;
    vector<int> nums;
    vector<int> numToIdx;
};