이분탐색

private static int binarySearch(int[] arr, int key) {
        int low = 0;
        int high = arr.length - 1;

        while (low <= high) {
            int middle = (low + high) / 2;
            if (key < arr[middle]) {
                high = middle - 1;
            } else if (key > arr[middle]) {
                low = middle + 1;
            } else return middle;
        }
        return -1;
    }

up&down 게임과 같은 원리