Counting Elements

Description

null

Solution(javascript)

/**
 * @param {number[]} arr
 * @return {number}
 */
var countElements = function(arr) {
    const set = new Set()
    
    arr.forEach(num => set.add(num))
    return arr.reduce((acc, num) => {
        if(set.has(num+1)){
            return acc + 1
        }
        return acc
    },0)
};