原题链接

解：KMP算法

class Solution {
public:int strStr(string s, string p) {if (p.empty()) return 0;int n = s.size(), m = p.size();s = ' ' + s, p = ' ' + p;vector<int> next(m + 1);for (int i = 2, j = 0; i <= m; i ++ ) {while (j && p[i] != p[j + 1]) j = next[j];if (p[i] == p[j + 1]) j ++ ;next[i] = j;}for (int i = 1, j = 0; i <= n; i ++ ) {while (j && s[i] != p[j + 1]) j = next[j];if (s[i] == p[j + 1]) j ++ ;if (j == m) return i - m;}return -1;}
};