代码随想录算法训练营第九天| 28. 找出字符串中第一个匹配项的下标

28. 找出字符串中第一个匹配项的下标：

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代码思路

KMP算法

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        n = len(needle)
        m = len(haystack)
        #构造next数组
        next = [0]
        k = 0
        for i in range(1,n):
            while k > 0 and needle[k] != needle[i]:
                k = next[k-1];
            if needle[k] == needle[i]:
                k += 1
            next.append(k)
            
        #主串位置判断
        j = 0
        for i in range(m):
            while j > 0 and haystack[i] != needle[j]:
                j = next[j-1]
            if haystack[i] == needle[j]:
                j += 1
            if j == n:
                return i-j+1
        return -1