Finding a ‘Lucky Integer’

Given an array, find the greatest value who’s numbers frequency is the same number as itself.

Example:

Input: [1,1,2,2,3,3]

Output: 2

It’s important to divide and conquere when working on algorithmic codes. This problem requires 3 steps.

1. Create a frequency table
2. Sort array in decending order
3. Iterate over the array until a hash key equals its value.

var findLucky = function(arr) {
    let hash = {}
    arr.forEach(num => {
        hash[num]? hash[num]++ : hash[num] = 1
    })
    arr.sort((a,b) => b - a)
    for(let i = 0; i < arr.length; i++){
        if(hash[arr[i]] === arr[i]){
            return arr[i]
        }
    }
    return -1
};

Further analysing, we can cut down on our runtime by eliminating the .sort method and instead using a temporary variable and returning it like so:

var findLucky = function(arr) {
    let hash = {}
    arr.forEach(num => {
        hash[num]? hash[num]++ : hash[num] = 1
    })
    let temp = -1
    for(let i = 0; i < arr.length; i++){
        if(hash[arr[i]] === arr[i]){
            if(arr[i] > temp){
                temp = arr[i]
            }
        }
    }
    return temp
};