c_cpp KMP的实施。复杂度O（n + m）

Posted 2021-05-14

/*==================================
 Title: Implementation of KMP Algorithm
 Complexity: O(n)
 Author : Sudipto Chandra
 ===================================*/
#include <bits/stdc++.h>
using namespace std;

const int SIZ = 100050;

int n, m;
char T[SIZ]; // given Text
char P[SIZ]; // Pattern to search
int pi[SIZ]; // LPS of pattern array. size=m
int lps[SIZ]; // LPS of pattern on text. size=n

// Build LPS array of the Pattern. Complexity: O(m)
// LPS[i] = Longest Prefix that is also a Suffix of P[0...i]
void computePrefix(int m)
{
     pi[0] = 0;
     int q = 0;
     for(int i = 1; i < m; ++i)
     {
         while(q > 0 && P[q] != P[i])
         {
             q = pi[q - 1];
         }

         if(P[q] == P[i])
         {
            q++;
         }
         pi[i] = q;
     }
}

// Find longest common prefix of the pattern
// occurs in the text. Complexity: O(n + m)
void KMP()
{
    n = strlen(T);
    m = strlen(P);

    computePrefix(m);

    int q = 0;
    for(int i = 0; i < n; ++i)
    {
        while(q > 0 && P[q] != T[i])
        {
            q = pi[q - 1];
        }
        if(P[q] == T[i])
        {
            q++;
        }
        lps[i] = q;
    }
}

void SHOW()
{
    vector<int> gap;

    printf("\n      ");
    for(int i = 0; i < n; ++i)
    {
        printf("%3c", T[i]);
    }
    printf("\nlps = ");
    for(int i = 0; i < n; ++i)
    {
        printf("%3d", lps[i]);
        if(lps[i] == m) gap.push_back(i - m + 1);
    }
    printf("\n------");
    for(int i = 0; i < n; ++i)
    {
        printf("---");
    }

    if(gap.empty())
    {
        puts("\n\n\tPattern not found.");
        return;
    }

    for(int k = 0; k < gap.size(); ++k)
    {
        int x = gap[k];

        printf("\n[@%3d]", x);
        if(x) printf("%*c", x * 3, ' ');
        for(int i = 0; i < m; ++i)
        {
            printf("%3c", P[i]);
        }
        printf("\n%*c", x * 3 + 6, ' ');
        for(int i = 0; i < m; ++i)
        {
            printf("%3d", pi[i]);
        }
        printf("\n------");
        for(int i = 0; i < n; ++i)
        {
            printf("---");
        }
    }

    puts("");
}

void RUN()
{
    printf("Text: ");
    gets(T);
    for(printf("\nPattern: "); gets(P); printf("\nPattern: "))
    {
        KMP();
        SHOW();
    }
}

int main()
{
    RUN();

    return 0;
}