Leetcode 1176

This is how the code works:

We sum up the first k elements.
Then compare whether sum is greater than upper or smaller than lower.
After comparing we can loop from k to len(calories).
Then we can take off the beggining of the subarray by subtracting calories[i - k]. And then we have to add the next element from the end of the subarray to the end to the subarray, we do this by adding calories[i]. We can do sum -= calories[i - k], and then sum += calories[i], or we can simplify these two lines into one line, sum -= calories[i - k] - calories[i] or sum += calories[i] - calories[i - k].
Then compare whether sum is greater than upper or smaller than lower.

The Code:

func dietPlanPerformance(calories []int, k int, lower int, upper int) int {
    sum := 0
    res := 0
    for i := 0; i < k; i++ {
        sum += calories[i]
    }
    res += comparison(sum, lower, upper)

    for i := k; i < len(calories); i++ {
        sum -= calories[i - k] - calories[i]
        res += comparison(sum, lower, upper)
    }
    return res
}

func comparison(sum int, lower int, upper int) int {
    if sum < lower {
        return -1
    } else if sum > upper {
        return 1
    }
    return 0
}