Implement strStr()

Description

Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

Solution(javascript)

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    function index(h, n, i) {
        if(h.length < n.length) return -1;
        if(h.substring(0, n.length) === n) return i;
        return index(h.slice(1), n, i+1);
    }
    return index(haystack, needle, 0);
};