LeetCode 28. 找出字符串中第一个匹配项的下标

题目链接：https://leetcode.cn/problems/find-the-index-of-the-first-occurrence-in-a-string/

C++代码如下：

class Solution {
public:
    int strStr(string haystack, string needle) {
        int n = haystack.size(), m = needle.size();
        haystack = ' ' + haystack; // 下标从1开始
        needle = ' ' + needle; // 下标从1开始

        vector<int> next(m + 1, 0);
        for (int i = 2, j = 0; i <= m; i++) {
            while (j && needle[i] != needle[j + 1]) j = next[j];
            if (needle[i] == needle[j + 1]) j++;
            next[i] = j;
        }
        
        for (int i = 1, j = 0; i <= n; i++) {
            while (j && haystack[i] != needle[j + 1]) j = next[j];
            if (haystack[i] == needle[j + 1]) j++;
            if (j == m) return i - m;
        }

        return -1;
    }
};