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
/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function (haystack, needle) {
    const slen = haystack.length;
    const plen = needle.length;
    if (slen == plen) {
        return haystack == needle ? 0 : -1;
    }
    for (let i = 0; i <= slen - plen; i++) {
        let j;
        for (j = 0; j < plen; j++) {
            if (haystack[i + j] != needle[j]) {
                break;
            }
        }
        if (j == plen) return i;
    }
    return -1;
};
