算法-kmp算法

• 前缀：aabaaf ：a,aa,aab,aaba,aabaa

• 后缀:f,af,aaf,baaf,abaaf

• 最长相等前后缀:a-0 aa-1 aab-0 aaba-1 aabaa-2 aabaaf-0

• 0 1 0 1 2 0就是aabaaf前缀表-next数组

//首先获取next i指向后缀末尾位置，j指向前缀末尾位置,遇到冲突看前一位


func getNext(s string,next []int){
    j :=0
    next[0] = j
    for i:=1;i<len(s);i++{
        for j>0&&s[i]!=s[j]{
            j = next[j-1]
        }
        if s[i]==s[j]{
            j++
        }
        next[i] = j
    }
}

func strStr(haystack string, needle string) int {
	n := len(needle)
	if n == 0 {
		return 0
	}
	j := 0
	next := make([]int, n)
	getNext(next, needle)
	for i := 0; i < len(haystack); i++ {
		for j > 0 && haystack[i] != needle[j] {
			j = next[j-1] // 回退到j的前一位
		}
		if haystack[i] == needle[j] {
			j++
		}
		if j == n {
			return i - n + 1
		}
	}
	return -1
}