#1426 - Counting Elements

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Problem Description

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.

Solution

/**
 * @param {number[]} arr
 * @return {number}
 */
var countElements = function(arr) {
  const set = new Set(arr);
  return arr.filter(num => set.has(num + 1)).length;
};