function closestNumbers(arr: number[]): number[] {
    /**
     * simple
     * 
     * 1. find [arr_sorted], sorted array of the [arr]
     * 2. find [min_diff], smallest difference between 2 adj element in [arr_sorted]
     * 3. loop [arr_sorted], only take pair that have difference smallest
     * 4. return
     */
    let arr_sorted = arr.sort((a, b) => a - b)

    let min_diff = null
    for (let i = 0; i < arr_sorted.length - 1; i++) {
        let pair = [arr_sorted[i], arr_sorted[i + 1]]
        let diff = Math.abs(pair[0] - pair[1])

        if (!min_diff || diff < min_diff) min_diff = diff
    }

    let pairs = []
    for (let i = 0; i < arr_sorted.length - 1; i++) {
        let pair = [arr_sorted[i], arr_sorted[i + 1]]
        let diff = Math.abs(pair[0] - pair[1])

        if (diff == min_diff) pairs.push(...pair)
    }

    return pairs;
}