Merge pull request #45 from nandikeshsingh/master

KMP-Algorithm

Author: prateekiiest

search/string_search/Knuth-Morris-Pratt_Algorithm.cpp

@@ -0,0 +1,80 @@
+//C++ code to find a pattern in a given text using KMP algorithm in linear time.
+#include<bits/stdc++.h>
+
+using namespace std;
+
+int main()
+{
+	string text;
+	string pattern;
+
+	cin>>text;//inputting text	
+	cout<<endl;
+
+	cin>>pattern;//inputting pattern
+	cout<<endl;
+
+	int pattern_length=pattern.size();//variable to store length of pattern string
+	int text_length=text.size();//variable to store length of text string     
+
+	if(pattern_length>text_length)//if pattern_length is greater than text it is not possible to find pattern in text.
+	{
+		cout<<"Not Found"<<endl;
+		exit(0);
+	}
+
+	int temp[pattern_length];//i th element in the temp array will store the length of the longest suffix which is equal to the prefix in pattern array upto i th index.
+
+	int i=1,j=0;
+
+	/*Suppose the patteren is ababa then temp[4] would store value 3 as the length of longest prefix(aba) which is equal 
+	to the longest suffix(aba) is 3 upto 4 th index in the pattern.The entire temp array would look like 0 0 1 2 3.*/
+	temp[0]=0;
+	while(i<pattern_length)//Building the temp array.
+	{
+		if(pattern[i]==pattern[j])
+		{
+			temp[i]=j+1;
+			i++;
+			j++;
+		}
+		else if(pattern[i]!=pattern[j] && j==0)
+		{
+			temp[i]=0;
+			i++;
+		}
+		else
+		{
+			j=temp[j-1];
+		}
+	}
+
+	i=0;
+	j=0;
+/*Suppose our text is abacd and our pattern is abad.During our first mismatch that is at d(j=3)in pattern and c(i=3)in text,
+unlike the naive string search we won't start again from i=1 in text and j=0 in pattern instead we look in temp[2] which will
+have value 1 and we set j to 1 and we will again start doing search from i=3 and j=1.*/
+	while(j<pattern_length && i<text_length)  //implementing the KMP alogorithm
+	{
+		if(pattern[j]==text[i])
+		{
+				j++;
+				i++;
+		}
+		else if(pattern[j]!=text[i] && j==0)
+		{
+			i++;
+		}
+		else
+		{
+			j=temp[j-1];
+		}
+	}
+
+	if(j==pattern_length)
+		cout<<"Match Found and the starting index is "<<i-pattern_length<<endl;
+	else 
+		cout<<"Not Found"<<endl;
+
+	return 0;
+}