Check If N and Its Double Exist — Binary Search or Hash Set

# Python — O(n) hash set
def checkIfExist(arr):
    seen = set()
    for x in arr:
        if 2*x in seen or (x%2==0 and x//2 in seen):
            return True
        seen.add(x)
    return False

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

# Python — sorting + binary search
import bisect
def checkIfExist(arr):
    arr.sort()
    n = len(arr)
    for i, x in enumerate(arr):
        target = 2*x
        j = bisect.bisect_left(arr, target)
        if j < n and arr[j] == target and j != i:
            return True
    return False