LESSON 121 OF 674 · ARRAYS - CHALLENGES

Drop the Ends in C#: A Middle Slice with GetRange

You get a List of integers nums. Return a List with every number except the first and the last.

The numbers in between keep their order. When nums has only two numbers, return an empty List.

Example 1:

Input: nums = [4, 8, 15, 16, 23]
Output: [8, 15, 16]
Explanation: 4 is the first number and 23 is the last.
Both are left out.

Example 2:

Input: nums = [9, 0, 9]
Output: [0]
Explanation: Both 9s are ends.
Only the 0 is between them.

Example 3:

Input: nums = [1, 2]
Output: []
Explanation: 1 and 2 are the ends.
Nothing is between them.

Constraints:

nums has 2 to 10 numbers.
Every number is an integer from -1000 to 1000.

STEP 1 OF 2

Guided

Ctrl + Enter to check

Drop The Ends in C#, explained

Problem overview

The method DropEnds receives a List<int> named nums and returns a new list with every number except the first and the last. The numbers in between keep their order. When nums has exactly two numbers, nothing is between them and the result is empty.

For [4, 8, 15, 16, 23] the ends are 4 and 23, so the result is [8, 15, 16]. For [9, 0, 9] the result is [0]. For [1, 2] it is []. The constraints say nums always has at least two numbers.

Approach

The numbers to keep form one block. It starts at index 1, right after the first number, and it is two items shorter than the list, because one item is missing at each end. So the block has nums.Count - 2 items.

GetRange(start, count) copies a block from a List<T> given its start index and its length. Together, the call is nums.GetRange(1, nums.Count - 2).

Check it on the three examples. With five numbers the count is 3, starting at index 1: the numbers at indexes 1, 2 and 3. With three numbers the count is 1. With two numbers the count is 0, which gives an empty list.

Step by step

Copy the middle. Call GetRange on nums with the start index 1 and the count nums.Count - 2. Store the new list in a List<int> named middle.
Return middle. For two numbers it is empty, which is the correct answer.

The solution in C#

class Solution {
    public List<int> DropEnds(List<int> nums) {
        List<int> middle = nums.GetRange(1, nums.Count - 2);
        return middle;
    }
}

On the three examples it produces these lists, printed as values separated by spaces (the empty list prints nothing):

8 15 16
0

The C# details worth knowing:

The size of a list is Count, a property with no parentheses. Length belongs to arrays and strings.
A count of zero is fine. GetRange(1, 0) on a two-item list returns an empty List<int>. The start index 1 is allowed because it is within the list.
A new list. The original nums keeps all its numbers, so the caller sees no change.
A shorter list would fail. On a list with a single number, the count would be -1 and GetRange throws an ArgumentOutOfRangeException. The constraints rule that out, so the lesson's code needs no guard.

Edge cases and common mistakes

Two numbers. The result is empty, because both are ends.
Three numbers. Exactly one number is left, as in [9, 0, 9]. Equal ends do not matter: the code works by position, not by value.
Count of nums.Count - 1. With a start of 1 this runs one item past the end and throws an ArgumentException. The start already skips one item, and the count has to leave out one more.
Start index 0. That keeps the first number and drops two from the end instead.
Using the second argument as an end index. GetRange(1, nums.Count - 1) would look like "from 1 up to the end" to a reader used to end indexes, but it is a count of items.

Testing

[4, 8, 15, 16, 23] gives [8, 15, 16].
[9, 0, 9] gives [0].
[1, 2] gives [].
[-5, 3, 3, -5] gives [3, 3].

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