题目：

题解：

class Solution {
public:int strStr(string haystack, string needle) {int n = haystack.size(), m = needle.size();if (m == 0) {return 0;}vector<int> pi(m);for (int i = 1, j = 0; i < m; i++) {while (j > 0 && needle[i] != needle[j]) {j = pi[j - 1];}if (needle[i] == needle[j]) {j++;}pi[i] = j;}for (int i = 0, j = 0; i < n; i++) {while (j > 0 && haystack[i] != needle[j]) {j = pi[j - 1];}if (haystack[i] == needle[j]) {j++;}if (j == m) {return i - m + 1;}}return -1;}
};