优化的朴素算法用于模式搜索

问题：我们在这里讨论了朴素字符串匹配算法。考虑一个模式的所有字符都不同的情况。我们可以修改原始的Naive String Matching算法，使其对这些类型的模式更好地工作吗?如果可以的话，原始算法会有什么变化?

解决方案：在原始的Naive String匹配算法中，我们总是将模式滑动1。当模式的所有字符都不同时，我们可以将模式滑动1以上。让我们看看如何做到这一点。当j个匹配后发生不匹配时，我们知道pattern的第一个字符将不匹配j个匹配的字符，因为pattern的所有字符都是不同的。因此，我们始终可以将模式滑动j而不丢失任何有效的移位。以下是针对特殊模式进行了优化的修改后的代码。

C++

/* C++ program for A modified Naive Pattern Searching 
algorithm that is optimized for the cases when all 
characters of pattern are different */
#include 
using namespace std;
  
/* A modified Naive Pettern Searching
algorithn that is optimized for the
cases when all characters of pattern are different */
void search(string pat, string txt) 
{ 
    int M = pat.size(); 
    int N = txt.size(); 
    int i = 0; 
  
    while (i <= N - M) 
    { 
        int j; 
  
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++) 
            if (txt[i + j] != pat[j]) 
                break; 
  
        if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1] 
        { 
            cout << "Pattern found at index " << i << endl; 
            i = i + M; 
        } 
        else if (j == 0) 
            i = i + 1; 
        else
            i = i + j; // slide the pattern by j 
    } 
} 
  
/* Driver code*/
int main() 
{ 
    string txt = "ABCEABCDABCEABCD"; 
    string pat = "ABCD"; 
    search(pat, txt); 
    return 0; 
} 
  
// This code is contributed by rathbhupendra

C

/* C program for A modified Naive Pattern Searching
  algorithm that is optimized for the cases when all
  characters of pattern are different */
#include
#include
  
/* A modified Naive Pettern Searching algorithn that is optimized
   for the cases when all characters of pattern are different */
void search(char pat[], char txt[])
{
    int M = strlen(pat);
    int N = strlen(txt);
    int i = 0;
  
    while (i <= N - M)
    {
        int j;
  
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++)
            if (txt[i+j] != pat[j])
                break;
  
        if (j == M)  // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
        {
           printf("Pattern found at index %d \n", i);
           i = i + M;
        }
        else if (j == 0)
           i = i + 1;
        else
           i = i + j; // slide the pattern by j
    }
}
  
/* Driver program to test above function */
int main()
{
   char txt[] = "ABCEABCDABCEABCD";
   char pat[] = "ABCD";
   search(pat, txt);
   return 0;
}

Java

/* Java program for A modified Naive Pattern Searching 
algorithm that is optimized for the cases when all 
characters of pattern are different */
  
class GFG
{
      
/* A modified Naive Pettern Searching
algorithn that is optimized for the
cases when all characters of pattern are different */
static void search(String pat, String txt) 
{ 
    int M = pat.length(); 
    int N = txt.length(); 
    int i = 0; 
  
    while (i <= N - M) 
    { 
        int j; 
  
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++) 
            if (txt.charAt(i + j) != pat.charAt(j)) 
                break; 
  
        if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1] 
        { 
            System.out.println("Pattern found at index "+i); 
            i = i + M; 
        } 
        else if (j == 0) 
            i = i + 1; 
        else
            i = i + j; // slide the pattern by j 
    } 
} 
  
/* Driver code*/
public static void main (String[] args) 
{
    String txt = "ABCEABCDABCEABCD"; 
    String pat = "ABCD"; 
    search(pat, txt); 
} 
}
  
// This code is contributed by chandan_jnu

Python

# Python program for A modified Naive Pattern Searching
# algorithm that is optimized for the cases when all
# characters of pattern are different
def search(pat, txt):
    M = len(pat)
    N = len(txt)
    i = 0
  
    while i <= N-M:
        # For current index i, check for pattern match
        for j in xrange(M):
            if txt[i+j] != pat[j]:
                break
            j += 1
  
        if j==M:    # if pat[0...M-1] = txt[i,i+1,...i+M-1]
            print "Pattern found at index " + str(i)
            i = i + M
        elif j==0:
            i = i + 1
        else:
            i = i+ j    # slide the pattern by j
  
# Driver program to test the above function
txt = "ABCEABCDABCEABCD"
pat = "ABCD"
search(pat, txt)
  
# This code is contributed by Bhavya Jain

C#

/* C# program for A modified Naive Pattern Searching 
algorithm that is optimized for the cases when all 
characters of pattern are different */
  
using System;
  
class GFG
{
      
/* A modified Naive Pettern Searching
algorithn that is optimized for the
cases when all characters of pattern are different */
static void search(string pat, string txt) 
{ 
    int M = pat.Length; 
    int N = txt.Length; 
    int i = 0; 
  
    while (i <= N - M) 
    { 
        int j; 
  
        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++) 
            if (txt[i + j] != pat[j]) 
                break; 
  
        if (j == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1] 
        { 
            Console.WriteLine("Pattern found at index "+i); 
            i = i + M; 
        } 
        else if (j == 0) 
            i = i + 1; 
        else
            i = i + j; // slide the pattern by j 
    } 
} 
  
/* Driver code*/
static void Main() 
{ 
    string txt = "ABCEABCDABCEABCD"; 
    string pat = "ABCD"; 
    search(pat, txt); 
} 
}
  
// This code is contributed by chandan_jnu

输出：

Pattern found at index 4
Pattern found at index 12