[leetcode #705] Design HashSet

Problem

Design a HashSet without using any built-in hash table libraries.

Implement MyHashSet class:

· void add(key) Inserts the value key into the HashSet.
· bool contains(key) Returns whether the value key exists in the HashSet or not.
· void remove(key) Removes the value key in the HashSet. If key does not exist in the HashSet, do nothing.

Example 1:

Input

["MyHashSet", "add", "add", "contains", "contains", "add", "contains", "remove", "contains"]
[[], [1], [2], [1], [3], [2], [2], [2], [2]]

Output

[null, null, null, true, false, null, true, null, false]

Explanation

MyHashSet myHashSet = new MyHashSet();
myHashSet.add(1);      // set = [1]
myHashSet.add(2);      // set = [1, 2]
myHashSet.contains(1); // return True
myHashSet.contains(3); // return False, (not found)
myHashSet.add(2);      // set = [1, 2]
myHashSet.contains(2); // return True
myHashSet.remove(2);   // set = [1]
myHashSet.contains(2); // return False, (already removed)

Constraints:

· 0 <= key <= 10⁶
· At most 104 calls will be made to add, remove, and contains.

Idea

HashSet을 구현하라는 문제다.

크기가 10⁶으로 제한되므로 boolean array를 쓰면 편하다. boolean array의 크기를 10⁶+1 만큼 잡아두고 key를 index로 하여 add면 true, remove면 false로 설정한다.

contains는 key를 index로 한 boolean array 값을 그대로 리턴하면 된다.

Solution

class MyHashSet {
    boolean[] flag;

    public MyHashSet() {
        flag = new boolean[1_000_001];
    }

    public void add(int key) {
        flag[key] = true;
    }

    public void remove(int key) {
        flag[key] = false;
    }

    public boolean contains(int key) {
        return flag[key];
    }
}

/**
 * Your MyHashSet object will be instantiated and called as such:
 * MyHashSet obj = new MyHashSet();
 * obj.add(key);
 * obj.remove(key);
 * boolean param_3 = obj.contains(key);
 */

Reference

https://leetcode.com/problems/design-hashset/