28. 实现 strStr() KMP 背

KMP模板

class Solution {
public:
    int strStr(string haystack, string needle) {
        int n = haystack.size();
        int m = needle.size();

        if (m == 0) {
            return 0;
        }

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

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

        return  -1;
    }
};