字符串 - 字符串搜索

28. 找出字符串中第一个匹配项的下标

方法一：前缀表统一减一

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

    const getNext = (needle) => {
        let next = [];
        let j = -1;
        next.push(j);

        for (let i = 1; i < needle.length; ++i) {
            while (j >= 0 && needle[i] !== needle[j + 1]){
                j = next[j];
            }
            if (needle[i] === needle[j + 1]) {
                j++;
            }
            next.push(j);
        }
        return next;
    }
    let next = getNext(needle);
    let j = -1;
    for (let i = 0; i < haystack.length; ++i) {
        while (j >= 0 && haystack[i] !== needle[j + 1]) {
            j = next[j];
        }
        if (haystack[i] === needle[j + 1]) {
            j++;
        }
        if (j === needle.length - 1) {
            return (i - needle.length + 1);
        }
    }
    
    return -1;
}

方法二：前缀表统一不减一

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

    const getNext = (needle) => {
        let next = [];
        let j = 0;
        next.push(j);

        for (let i = 1; i < needle.length; ++i) {
            while (j > 0 && needle[i] !== needle[j])
                j = next[j - 1];
            if (needle[i] === needle[j])
                j++;
            next.push(j);
        }

        return next;
    }

    let next = getNext(needle);
    let j = 0;
    for (let i = 0; i < haystack.length; ++i) {
        while (j > 0 && haystack[i] !== needle[j])
            j = next[j - 1];
        if (haystack[i] === needle[j])
            j++;
        if (j === needle.length)
            return (i - needle.length + 1);
    }

    return -1;
};