2336. Smallest Number in Infinite Set

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# 題目敘述

You have a set which contains all positive integers [1, 2, 3, 4, 5, ...] .

Implement the SmallestInfiniteSet class:

• SmallestInfiniteSet() Initializes the SmallestInfiniteSet object to contain all positive integers.
• int popSmallest() Removes and returns the smallest integer contained in the infinite set.
• void addBack(int num) Adds a positive integer num back into the infinite set, if it is not already in the infinite set.

# Example 1:

Input
["SmallestInfiniteSet", "addBack", "popSmallest", "popSmallest", "popSmallest", "addBack", "popSmallest", "popSmallest", "popSmallest"]
[[], [2], [], [], [], [1], [], [], []]
Output
[null, null, 1, 2, 3, null, 1, 4, 5]

Explanation
SmallestInfiniteSet smallestInfiniteSet = new SmallestInfiniteSet();
smallestInfiniteSet.addBack(2); // 2 is already in the set, so no change is made.
smallestInfiniteSet.popSmallest(); // return 1, since 1 is the smallest number, and remove it from the set.
smallestInfiniteSet.popSmallest(); // return 2, and remove it from the set.
smallestInfiniteSet.popSmallest(); // return 3, and remove it from the set.
smallestInfiniteSet.addBack(1); // 1 is added back to the set.
smallestInfiniteSet.popSmallest(); // return 1, since 1 was added back to the set and
// is the smallest number, and remove it from the set.
smallestInfiniteSet.popSmallest(); // return 4, and remove it from the set.
smallestInfiniteSet.popSmallest(); // return 5, and remove it from the set.

# 解題思路

# Solution

import java.util.PriorityQueue;

class SmallestInfiniteSet {

    PriorityQueue<Integer> pQ;

    public SmallestInfiniteSet() {

        pQ = new PriorityQueue<>();

        for(int i = 1; i <= 1000; i++){

            pQ.add(i);

        }

    }

    public int popSmallest() {

        int num = pQ.poll();

        return num;

    }

    public void addBack(int num) {

        if(!pQ.contains(num)){

            pQ.add(num);

        }

    }

}

/**

 * Your SmallestInfiniteSet object will be instantiated and called as such:

 * SmallestInfiniteSet obj = new SmallestInfiniteSet();

 * int param_1 = obj.popSmallest();

 * obj.addBack(num);

 */