Implement strStr() | LeetCode 28

[Written explanation will be added soon]

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Here is the full implementation.

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
let strStr = function(haystack, needle) {
    if (!needle) {
      return 0;
    }

    let pointer2 = needle.length;

    for (let pointer1 = 0; pointer1 < (haystack.length - needle.length) + 1; pointer1++) {
      let substring = haystack.substring(pointer1, pointer2);
      if (substring === needle) {
        return pointer1;
      }
      pointer2++;
    }

    return -1;
};