10. Longest Happy Prefix (HASHING)

Hashing/C++/10. Longest Happy Prefix (HASHING).cpp

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+
+// Longest Happy Prefix (HASHING)
+
+/*
+
+Question : We will be given a string s, and we need to compute the longest prefix which is also a suffix.
+
+Algorithm :
+	In the we use to precompute the an array, which tells us about the longest prefix which is also suffix.
+	so we will find the that array and form a string of length equals to last element in vector, and will return that string
+*/
+
+
+#include<bits/stdc++.h>
+using namespace std;
+
+// main function defined to get the length of longest happy prefix
+string longest_happy_prefix(string &input_string)
+{
+	// vector created to store the length of longest happy prefix from from each index 
+    vector<int> temp;
+	for(int i=0;i<input_string.length();i++)
+	{
+		temp.push_back(0);
+	}
+
+    // temporary variable taken to keep track od index
+    int idx = 0;
+    for(int i = 1; i < input_string.length();i+=0)// we loop through length of input string 
+    {
+    	// if character at index i matches with character at index idx
+        if(input_string[i] == input_string[idx])
+        {
+            idx++;// we increase the value of idx 
+            temp[i] = idx;// insert that idx value in temp vector, since this is the length of prefix which is suffix also
+            i++;
+        }
+        // else is characters don't matches, then we need to update the idx value to the new value
+        else
+        {
+        	if(idx==0)
+        	{
+        		i++;//incremented i
+			}
+			else
+            {
+            	idx = temp[idx-1];// idx updated
+			}
+        }
+    }
+
+    // now the length of the longest happy prefix is the last element in temp vector,
+    // so we form a string pf that length
+    string ans = "";
+    for(int i=0;i<temp.back();i++)
+    {
+    	ans += input_string[i];
+	}
+
+	// return the longest happy prefix
+    return ans;
+}
+
+int main()
+{
+	// declared two variable n and m, for size of both array
+	string input_string;
+	// taking input of both array size
+	cin>>input_string;
+
+	// function called for finding the maximum lenght of repeated subarray
+	cout<<"The Longest Happy Prefix is  :  "<<longest_happy_prefix(input_string)<<endl;
+
+	return 0;
+}
+
+/*
+
+Time Complexity : O(n)
+Space Complexity : O(n)
+
+Sample testcase :
+Input 1 :
+	anyonecananyone
+Output 1:
+	The Maximum length of repeated subarray is  :  anyone
+Explaination :
+	In the input string "anyonecananyone", the length of longest happy prefix is "anyone", because we can see that in the inputstring substring "anyone" 
+	is present from index 0 to 5 and other is from 9 to 14.
+	Thus we can say the substring "anyone" may be one of the happy prefix.
+	Now if we add one more character in "anyone", like we take "anyonec", we can clearly see that this substing is not suffix also.
+	So "anyone" substring should be the longest happy prefix.
+
+Input 2 :
+	luckyfool
+Output 1:
+	The Maximum length of repeated subarray is  :  l
+Explaination :
+	In the input string "luckyfool", the length of longest happy prefix is "l", because we can see that in the inputstring substring "l" 
+	is present as the first letter and the last letter.
+	Thus we can say the substring "l" may be one of the happy prefix.
+	Now if we add one more character in "l", like we take "lu", we can clearly see that this substing is not present as suffix also.
+	So "l" substring should be the longest happy prefix.
+
+*/
+