LeetCode 2336. Smallest Number in Infinite Set Solution in Java, C++, Python & More | Explanation + Code

Description

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

Solution 1: Ordered Set + Simulation

We note that the range of elements in the set given by the problem is [1, 1000], and the operations we need to support are:

• popSmallest: Pop the smallest element from the set
• addBack: Add an element back to the set

Therefore, we can use an ordered set to simulate this. Let's denote the ordered set as s, and the elements in the set as s₁, s₂, …, s_n, where n is the number of elements in the ordered set. In this problem, n \le 1000.

During initialization, we add all elements in [1, 1000] to the ordered set. The time complexity is O(n × log n).

In the popSmallest operation, we just need to pop the first element from the ordered set. The time complexity for a single operation is O(log n).

In the addBack operation, we just need to add the element back to the ordered set. The time complexity for a single operation is O(log n).

The space complexity is O(n).

PythonJavaC++GoTypeScriptRust

class SmallestInfiniteSet: def __init__(self): self.s = SortedSet(range(1, 1001)) def popSmallest(self) -> int: x = self.s[0] self.s.remove(x) return x def addBack(self, num: int) -> None: self.s.add(num) # Your SmallestInfiniteSet object will be instantiated and called as such: # obj = SmallestInfiniteSet() # param_1 = obj.popSmallest() # obj.addBack(num)(code-box)

class SmallestInfiniteSet { private TreeSet<Integer> s = new TreeSet<>(); public SmallestInfiniteSet() { for (int i = 1; i <= 1000; ++i) { s.add(i); } } public int popSmallest() { return s.pollFirst(); } public void addBack(int num) { s.add(num); } } /** * Your SmallestInfiniteSet object will be instantiated and called as such: * SmallestInfiniteSet obj = new SmallestInfiniteSet(); * int param_1 = obj.popSmallest(); * obj.addBack(num); */(code-box)

class SmallestInfiniteSet { public: SmallestInfiniteSet() { for (int i = 1; i <= 1000; ++i) { s.insert(i); } } int popSmallest() { int x = *s.begin(); s.erase(s.begin()); return x; } void addBack(int num) { s.insert(num); } private: set<int> s; }; /** * Your SmallestInfiniteSet object will be instantiated and called as such: * SmallestInfiniteSet* obj = new SmallestInfiniteSet(); * int param_1 = obj->popSmallest(); * obj->addBack(num); */(code-box)

type SmallestInfiniteSet struct { s *treemap.Map } func Constructor() SmallestInfiniteSet { s := treemap.NewWithIntComparator() for i := 1; i <= 1000; i++ { s.Put(i, nil) } return SmallestInfiniteSet{s} } func (this *SmallestInfiniteSet) PopSmallest() int { x, _ := this.s.Min() this.s.Remove(x.(int)) return x.(int) } func (this *SmallestInfiniteSet) AddBack(num int) { this.s.Put(num, nil) } /** * Your SmallestInfiniteSet object will be instantiated and called as such: * obj := Constructor(); * param_1 := obj.PopSmallest(); * obj.AddBack(num); */(code-box)

class SmallestInfiniteSet { private pq = new MinPriorityQueue<number>(); private s = new Set<number>(); constructor() { for (let i = 1; i <= 1000; i++) { this.pq.enqueue(i); this.s.add(i); } } popSmallest(): number { const x = this.pq.dequeue(); this.s.delete(x); return x; } addBack(num: number): void { if (!this.s.has(num)) { this.pq.enqueue(num); this.s.add(num); } } } /** * Your SmallestInfiniteSet object will be instantiated and called as such: * var obj = new SmallestInfiniteSet() * var param_1 = obj.popSmallest() * obj.addBack(num) */(code-box)

use std::collections::BTreeSet; struct SmallestInfiniteSet { s: BTreeSet<i32>, } impl SmallestInfiniteSet { fn new() -> Self { let mut set = BTreeSet::new(); for i in 1..=1000 { set.insert(i); } SmallestInfiniteSet { s: set } } fn pop_smallest(&mut self) -> i32 { let x = *self.s.iter().next().unwrap(); self.s.remove(&x); x } fn add_back(&mut self, num: i32) { self.s.insert(num); } }(code-box)