Create LongestPalindromeSubstring.cpp

Cpp/LongestPalindromeSubstring.cpp

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+// A O(n^2) time and O(1) space program to
+// find the longest palindromic substring
+// easy to understand as compared to previous version.
+#include <bits/stdc++.h>
+using namespace std;
+
+// A utility function to print
+// a substring str[low..high]
+// This function prints the
+// longest palindrome substring (LPS)
+// of str[]. It also returns the
+// length of the longest palindrome
+int longestPalSubstr(string str)
+{
+	int n = str.size(); // calculating size of string
+	if (n < 2)
+		return n; // if string is empty then size will be 0.
+				// if n==1 then, answer will be 1(single
+				// character will always palindrome)
+
+	int maxLength = 1, start = 0;
+	int low, high;
+	for (int i = 0; i < n; i++) {
+		low = i - 1;
+		high = i + 1;
+		while (high < n
+			&& str[high] == str[i]) // increment 'high'
+			high++;
+
+		while (low >= 0
+			&& str[low] == str[i]) // decrement 'low'
+			low--;
+
+		while (low >= 0 && high < n
+			&& str[low] == str[high]) {
+			low--;
+			high++;
+		}
+
+		int length = high - low - 1;
+		if (maxLength < length) {
+			maxLength = length;
+			start = low + 1;
+		}
+	}
+
+	cout << "Longest palindrome substring is: ";
+	cout << str.substr(start, maxLength);
+	return maxLength;
+}
+
+// Driver program to test above functions
+int main()
+{
+	string str = "forgeeksskeegfor";
+	cout << "\nLength is: " << longestPalSubstr(str)
+		<< endl;
+	return 0;
+}