README.md

2336. Smallest Number in Infinite Set

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.

Constraints:

• 1 <= num <= 1000
• At most 1000 calls will be made in total to popSmallest and addBack.

Solutions (Rust)

1. Solution

use std::collections::BinaryHeap;
use std::collections::HashSet;

struct SmallestInfiniteSet {
    min_infinite: i32,
    heap: BinaryHeap<i32>,
    set: HashSet<i32>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl SmallestInfiniteSet {
    fn new() -> Self {
        Self {
            min_infinite: 1,
            heap: BinaryHeap::new(),
            set: HashSet::new(),
        }
    }

    fn pop_smallest(&mut self) -> i32 {
        match self.heap.pop() {
            Some(x) => self.set.take(&(-x)).unwrap(),
            None => {
                self.min_infinite += 1;
                self.min_infinite - 1
            }
        }
    }

    fn add_back(&mut self, num: i32) {
        if num < self.min_infinite && self.set.insert(num) {
            self.heap.push(-num);
        }
    }
}

/**
 * Your SmallestInfiniteSet object will be instantiated and called as such:
 * let obj = SmallestInfiniteSet::new();
 * let ret_1: i32 = obj.pop_smallest();
 * obj.add_back(num);
 */