KMP算法实现

zslInSz

于 2014-03-13 21:24:26 发布

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分类专栏：算法学习

本文链接：https://blog.youkuaiyun.com/wyyzsl212328/article/details/21190561

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本文介绍了一种改进的字符串搜索算法——KMP算法，并提供了详细的C++实现代码。该算法通过预处理模式串生成部分匹配表来提高匹配效率，避免了传统模式匹配中不必要的回溯。

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#include <stdlib.h>
#include <stdio.h>
#include <string>
using namespace std;

int * cover(string &pattern)
{
    /*pattern = "abcdab"*/
    int size = pattern.size();
    int *p = new int[size];
    int length = 0,r = size;
    string str1;
    if (!p)
    {
        printf("new error\n");
        exit(1);
    }

    for (int i = 1;i < size+1; i++)
    {
        str1 = pattern.substr(0, i);
        length = i - 1;
        r = 1;
        while (str1.substr(0,length) != str1.substr(r, length))
        {
            length--;
            r++;
        }
        p[i - 1] = length;
    }

    return p;
}


int kmp(string &str, string &pattern)
{
    int r = -1, i = 0;
    unsigned int   pos;
    int *p = cover(pattern);
    int length = str.size() - pattern.size();

    for (int m = 0; m < pattern.size(); m++)
    {
        printf("pattern[%d] = %4d\n", m, p[m]);
    }

    for (i; i < length; i++)
    {
        pos = 0;
        while(pos < pattern.size())
        {
            if (str[i+pos] == pattern[pos])
            {
                pos++;
            }

            else
            {
                i += p[pos - 1];
                break;
            }
        }
        if (pos == (pattern.size() - 1))
        {
            delete[] p;
            return r = i;
        }
    }

    delete[] p;
    return r;
}


int main()
{
    string str = "abcabcabdfg";
    string pattern = "abcabd";
    int pos = kmp(str, pattern);
    printf("%d\n", pos);
    return 0;
}