实现 strStr()

输入: haystack = "hello", needle = "ll"
输出: 2

输入: haystack = "aaaaa", needle = "bba"
输出: -1

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    if (needle === '') return 0
    for(let i = 0, l = haystack.length; i < l; i++) {
        if (haystack[i] === needle[0]) {
            if (needle === haystack.substring(i, i + needle.length)) {
                return i
            }
        }
    }
    return -1
};

var strStr = function(haystack, needle) {
    const n = haystack.length, l = needle.length
    if (l === 0) {
        return 0
    }
    let pn = 0
    while (pn < n - l + 1) {
        while (pn < n - l + 1 && haystack.charAt(pn) !== needle.charAt(0)){
            ++pn
        }
        let curr = 0, pl = 0
        while (pl < l && pn < n && haystack.charAt(pn) === needle.charAt(pl)) {
            ++pn
            ++pl
            ++curr
        }
        if (curr === l) {
            return pn - l
        }
        pn = pn - curr + 1
    }
    return -1
}