数据结构与算法：python语言描述学习笔记Part4_kmp算法及改进

# -*- coding:utf-8 -*-

# 构造next数组函数

def gen_pnext(p):

i, k, m = 0, - 1, len(p)

pnext = [- 1]*m # 初始数组元素全为-1

while i < m- 1: # 生成下一个pnext元素值

if k == - 1 or p[i] == p[k]:

i, k = i+ 1, k+ 1

pnext[i] = k # 设置pnext元素

else:

k = pnext[k] # 退到更短相同前缀

return pnext

# KMP算法的改进

def gen_pnext_improve(p):

'''改进一点的版本，在下一跳字符和当前相同时，跳到下一跳的下一跳，跳的更远'''

i, k, m = 0, - 1, len(p)

pnext = [- 1] * m

while i < m- 1:

if k == - 1 or p[i] == p[k]:

i, k = i+ 1, k+ 1

if p[i] == p[k]:

pnext[i] = pnext[k]

else:

pnext[i] = k

else:

k = pnext[k]

return pnext

# 自己的神他喵麻烦构建next数组版本

def MINE_gen_pnext(p):

'''生成针对p中各个未知的i的下一跳位置表'''

pnext = [- 1, 0]

pnext=[]

count = 0

for j in range( 0, len(p)+ 1):

ps = p[ 0:j]

if ps == '':

continue

for i in range(len(ps), 1, - 1):

if ps[ 0: i- 1] == ps[-i+ 1:]:

pnext.append(len(ps[ 0: i- 1]))

if count

count += 1

else:

pnext.append( 0)

count += 1

for i in range(len(pnext)- 1, 0, - 1):

pnext[i] = pnext[i- 1]

pnext[ 0] = - 1

return pnext

# kmp匹配主函数

def matching_KMP(t, p):

'''KMP Main fuction, t for target, p for pattern'''

i, j = 0, 0

n, m = len(t), len(p)

pnext = gen_pnext(p)

while j < n and i < m: # i == m说明找到了匹配

if i == - 1 or t[j] == p[i]: # 刚开始或者字符匹配时，考虑p中的下一字符

j, i = j+ 1, i+ 1

else: # 字符不匹配，则考虑pnext对应的下一字符

i = pnext[i]

if i == m: # 找到匹配，返回其下标

return j - i

return - 1 # 无匹配

print matching_KMP( 'ABCDEFG', 'EFG')