< Back 1426. Counting Elements We have an integer array. For each integer, x, in the array, how many times does x + 1 appear? Count duplicates. To easily solve this, we use a set() to conduct a lookup in O(1) time, placing all values of the input array into the set. Then, we just iterate through the entire list, checking each value, x. If x + 1 is in the set we created, we increase the count. The solution is as follows: class Solution: def countElements(self, arr: List[int]) -> int: set_arr = set(arr) count = 0 for num in arr: if num + 1 in set_arr: count += 1 return count _ Time Complexity: O(n) - We have to iterate through the entire input array. _ Space Complexity: O(n) - We have to create a set to store the input array.