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kmp.cpp
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78 lines (67 loc) · 1.76 KB
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/**
* Knuth-Morris-Pratt (KMP) Algorithm
*
* Problem: Find occurrences of a pattern P in a text T.
* Naive Approach: O(N * M).
* KMP Approach: O(N + M) using a prefix function (LPS array).
*
* LPS[i] = Longest Proper Prefix which is also a Suffix for pattern[0...i].
*/
#include <iostream>
#include <vector>
#include <string>
using namespace std;
// Compute LPS Array
vector<int> computeLPS(string pattern) {
int m = pattern.length();
vector<int> lps(m);
int len = 0; // Length of previous longest prefix suffix
lps[0] = 0; // Always 0
int i = 1;
while (i < m) {
if (pattern[i] == pattern[len]) {
len++;
lps[i] = len;
i++;
} else {
if (len != 0) {
len = lps[len - 1]; // Fallback to previous prefix
} else {
lps[i] = 0;
i++;
}
}
}
return lps;
}
// KMP Search
void KMPSearch(string text, string pattern) {
int n = text.length();
int m = pattern.length();
vector<int> lps = computeLPS(pattern);
int i = 0; // index for text
int j = 0; // index for pattern
while (i < n) {
if (pattern[j] == text[i]) {
j++;
i++;
}
if (j == m) {
cout << "Pattern found at index " << i - j << "\n";
j = lps[j - 1]; // Look for next match
} else if (i < n && pattern[j] != text[i]) {
if (j != 0)
j = lps[j - 1];
else
i = i + 1;
}
}
}
int main() {
string text = "ABABDABACDABABCABAB";
string pattern = "ABABCABAB";
cout << "Text: " << text << "\n";
cout << "Pattern: " << pattern << "\n";
KMPSearch(text, pattern);
return 0;
}