Implement strStr()

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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.

Constraints:

• 1 <= haystack.length, needle.length <= 104

• haystack and needle consist of only lowercase English characters.

You can solve this problem using the indexOf method available for strings in JavaScript. Here's how:

function strStr(haystack, needle) {
    return haystack.indexOf(needle);
}

// Test cases
console.log(strStr("sadbutsad", "sad"));   // Output: 0
console.log(strStr("leetcode", "leeto"));  // Output: -1

This implementation utilizes the indexOf method, which returns the index of the first occurrence of the specified substring (needle) within the string (haystack). If the substring is not found, it returns -1.