字符串匹配 - KMP算法

实现 strStr() 函数。给定一个 haystack 字符串和一个 needle 字符串，在 haystack 字符串中找出 needle 字符串出现的第一个位置 (从0开始)。如果不存在，则返回  -1

• 示例1:

输入: haystack = "hello", needle = "ll"
输出: 2

• 示例2:

输入: haystack = "aaaaa", needle = "bba"
输出: -1

题解

阮一峰： http://www.ruanyifeng.com/blog/2013/05/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm.html
写的非常不错，容易懂，博主这边就不做多解释，这边贴一下例子就行

实现代码

• Python

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        if len(needle) == 0:
            return 0
        next = self.next_list(needle)
        i = 0
        j = 0
        ans = -1
        n = len(haystack)
        while i < n:
            if haystack[i] == needle[j] or j == -1:
                i += 1
                j += 1
            else:
                j = next[j]
            if j == len(needle):
                ans = i - j
                break
        return ans

    # 用来生成next数组，判别最长相同前后缀
    def next_list(self, needle):
        n = len(needle)
        k = -1
        i = 0
        next = [0 for i in range(n)]
        next[0] = -1
        while i < n-1:
            if needle[i] == needle[k] or k == -1:
                i += 1
                k += 1
                next[i] = k
            else:
                k = next[k]
        return next