1346. Check If N and Its Double Exist

https://leetcode.com/problems/check-if-n-and-its-double-exist/

Given an array arr of integers, check if there exists two integers N and M such that N is the double of M ( i.e. N = 2 * M).

More formally check if there exists two indices i and j such that :

• i != j

• 0 <= i, j < arr.length

• arr[i] == 2 * arr[j]

数组中是否存在某元素是另一个元素的二倍。遍历，hash set记录已经出现过的元素，检查set里是否有当前元素两倍或1/2的元素（且能整除）。

class Solution {
public:
    bool checkIfExist(vector<int>& arr) {
        unordered_set<int> s;
        for (const auto a : arr) {
            if (s.count(a * 2) || a % 2 == 0 && s.count(a / 2)) return true; 
            s.insert(a);
        }
        return false;
    }
};