Find the Index of the First Occurrence in a String
<p>Given two strings <code>needle</code> and <code>haystack</code>, return the index of the first occurrence of <code>needle</code> in <code>haystack</code>, or <code>-1</code> if <code>needle</code> is not part of <code>haystack</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> haystack = &quot;sadbutsad&quot;, needle = &quot;sad&quot;
<strong>Output:</strong> 0
<strong>Explanation:</strong> &quot;sad&quot; occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> haystack = &quot;leetcode&quot;, needle = &quot;leeto&quot;
<strong>Output:</strong> -1
<strong>Explanation:</strong> &quot;leeto&quot; did not occur in &quot;leetcode&quot;, so we return -1.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= haystack.length, needle.length &lt;= 10<sup>4</sup></code></li>
	<li><code>haystack</code> and <code>needle</code> consist of only lowercase English characters.</li>
</ul>
Two Pointers; String; String Matching
public class Solution {
    public int StrStr(string haystack, string needle) {
        for (var i = 0; i < haystack.Length - needle.Length + 1; ++i)
        {
            var j = 0;
            for (; j < needle.Length; ++j)
            {
                if (haystack[i + j] != needle[j]) break;
            }
            if (j == needle.Length) return i;
        }
        return -1;
    }
}
