Find the number of ways to represent n as a sum of two integers with boundaries

问题

I am playing around codefight, but I am really stuck to the following efficient issue.

Problem:
Given integers n, l and r, find the number of ways to represent n as a sum of two integers A and B such that l ≤ A ≤ B ≤ r.

Example:
For n = 6, l = 2 and r = 4, the output should be countSumOfTwoRepresentations2(n, l, r) = 2. There are just two ways to write 6 as A + B, where 2 ≤ A ≤ B ≤ 4: 6 = 2 + 4 and 6 = 3 + 3.

Here is my code. It passes all the unit tests but it failing in the hidden ones. Can someone direct me somehow? Thanks in advance.

public static int countSumOfTwoRepresentations2(int n, int l, int r) {
    int nrOfWays = 0;
    for(int i=l;i<=r;i++)
    {
        for(int j=i;j<=r;j++)
        {
            if(i+j==n)
                nrOfWays++;
        }
    }
    return nrOfWays;

}

回答1:

Well, there's no need to make so huge calculations... It's easy to calculate:

public static int count(int n, int l, int r) {
    if (l > n/2)
        return 0;
    return Math.min(n/2 - l, r - n/2) + ((n%2 == 1) ? 0 : 1);
}

Passes all my tests so far. For positives and negatives as well.

回答2:

In java:

int countSumOfTwoRepresentations2(int n, int l, int r)
{
     return Math.max(0,Math.min(n/2-l,r-n/2)+(n+1)%2);
}

In Python3:

def countSumOfTwoRepresentations2(n, l, r):
    return max(0,min(n//2-l,r-n//2)+(n+1)%2)

回答3:

int countSumOfTwoRepresentations(int n, int l, int r)
  {
    int r1 = 0;
    if (n > l + r || n < 2 * l)
        return 0;
    r1 = n - l;
    if ((r1 - l) % 2 == 0)
        return (r1 - l) / 2 + 1;
    else
        return (r1 - l + 1) / 2;
}

来源：https://stackoverflow.com/questions/39860021/find-the-number-of-ways-to-represent-n-as-a-sum-of-two-integers-with-boundaries