/*
 * 1426. Counting Elements
 * Difficulty: Easy
 * https://leetcode.com/problems/counting-elements/
 * 
 * ──────────────────────────────────────────────────
 * 
 * Given an integer array arr, count how many elements x there are, such
 * that x + 1 is also in arr. If there are duplicates in arr, count them
 * separately.
 * 
 *  
 * 
 * Example 1:
 * 
 * Input: arr = [1,2,3]
 * Output: 2
 * Explanation: 1 and 2 are counted cause 2 and 3 are in arr.
 * 
 * Example 2:
 * 
 * Input: arr = [1,1,3,3,5,5,7,7]
 * Output: 0
 * Explanation: No numbers are counted, cause there is no 2, 4, 6, or 8
 * in arr.
 * 
 *  
 * 
 * Constraints:
 * 
 * 	• 1 <= arr.length <= 1000
 * 
 * 	• 0 <= arr[i] <= 1000
*/

/**
 * @param {number[]} arr
 * @return {number}
 */
var countElements = function(arr) {
  let arrSet = new Set(arr);
  let count = 0; 

  for (let n of arr) {
    let sum = n + 1;
    if (arrSet.has(sum)) {
      count++;
    }
  }
  return count;
};