LeetCode 1346. Check If N and Its Double Exist

Given an array arr of integers, check if there exist two indices i and j such that :

• i != j
• 0 <= i, j < arr.length
• arr[i] == 2 * arr[j]

Example 1:

Input: arr = [10,2,5,3]
Output: true
Explanation: For i = 0 and j = 2, arr[i] == 10 == 2 * 5 == 2 * arr[j]

Example 2:

Input: arr = [3,1,7,11]
Output: false
Explanation: There is no i and j that satisfy the conditions.

Constraints:

• 2 <= arr.length <= 500
• -103 <= arr[i] <= 103

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就是求一个数组里有没有一个数是另一个数的两倍。

在做了一堆2 pointers以后遇到这道题居然差点不会做了，直觉告诉我是需要用点hashset但是不确定，就看了眼solutions的title，果然是set，于是就秒写出来了。想到了就tooo easy了。

class Solution {
    public boolean checkIfExist(int[] arr) {
        Set set = new HashSet<>();
        for (int i : arr) {
            if (set.contains(2 * i) || (i % 2 == 0 && set.contains(i / 2))) {
                return true;
            }
            set.add(i);
        }
        return false;
    }
}