KMP算法

KMP算法理解

设模式串为pattern[0,1,2…p-1]

核心思想

• 找到模式子串pattern[0,1,2…k-1]的真后缀同时也是pattern[0,1,2…p-1]的最大前缀的长度（pattern的总长度是p，0<=k<=p）,存入pi数组。
• 利用pi数组来避免模式串和主串的不必要的比较，提高效率

KMP算法C++代码

#include <iostream>#include <string>#include <vector>using namespace std;//得到模式串的pi数组void get_pi(string pattern, int plen, int *pi){    if (pattern.size() == 0) return;    int k = -1;    pi[0] = -1;    for (int i = 1; i < plen; i++){        while (k > -1 && pattern[i] != pattern[k + 1])            k = pi[k];        if (pattern[i] == pattern[k + 1])            k = k + 1;        pi[i] = k;    }    return;}//找到所有的模式串在主串中的位置，并存入vector中返回vector<int> search_substr(string str, string pattern){    vector<int> subindex;    int plen = pattern.length(), strlen = str.length(), q = -1;    int *pi = new int[plen];    get_pi(pattern, plen, pi);    for (int i = 0; i < strlen; i++){        while (q > -1 && str[i] != pattern[q + 1])            q = pi[q];        if (str[i] == pattern[q + 1])            q = q + 1;        if (q == plen - 1)        {            subindex.push_back(i-plen+1);            q = pi[q];        }    }    delete[] pi;    return subindex;}//主函数int main(){    string str = "ababcabcacbabcac";    string pattern = "abcac";    vector<int> subindex = search_substr(str, pattern);    for (auto iter : subindex){        cout << iter<< endl;    }    getchar();    return 0;}