题目：

题解：

func strStr(haystack, needle string) int {n, m := len(haystack), len(needle)if m == 0 {return 0}pi := make([]int, m)for i, j := 1, 0; i < m; i++ {for j > 0 && needle[i] != needle[j] {j = pi[j-1]}if needle[i] == needle[j] {j++}pi[i] = j}for i, j := 0, 0; i < n; i++ {for 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
}