C# LeetCode 28: Find the Index of the the First Occurrence in a String

Problem

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "sadbutsad", needle = "sad"
Output: 0
*Explanation: *"sad" occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.
Example 2:

Input: haystack = "leetcode", needle = "leeto"
Output: -1
*Explanation: *"leeto" did not occur in "leetcode", so we return -1.

Code:

Iteration 1: (Poor) - Tried without built in functions

public int StrStr(string haystack, string needle) {
    if (haystack.Length < needle.Length) return -1;

    for (var i = 0; i < haystack.Length; i++)
    {
      if (haystack[i..].StartsWith(needle)) return i;
    }

    return -1;
}

Why is this bad?

Well, there's unnecessary allocation of strings. haystack[i..] creates a new string on every iteration, which is costly for large strings.

Should handle empty needle (LeetCode requires returning 0).

Should handle empty haystack gracefully.

In C# it's just unnecessary

Iteration 2: (More Efficient)

public int StrStr(string haystack, string needle) {
    return haystack.IndexOf(needle, StringComparison.Ordinal);
}

Approach

Bosh, simply use C#'s built in IndexOf method, which will return the index of one character / string in another. If it doesn't exist it will return -1 (not found)