From edc5b47f68c46a927b733a976db4d780525c596d Mon Sep 17 00:00:00 2001
From: Sergey Popov <sergobot@protonmail.com>
Date: Tue, 24 Oct 2017 18:55:26 +0300
Subject: [PATCH] Fix #540: Add Longest Palindromic Substring [Go] (#564)

* Added Longest Palindromic Substring [Go]

* Adjust code style and add entry to README
---
 README.md                                     |  2 +-
 .../longest_palindromic_substring.go          | 46 +++++++++++++++++++
 2 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 longest_palindromic_substring/longest_palindromic_substring.go

diff --git a/README.md b/README.md
index f91eb284..b1770895 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,7 @@ Community (college) maintained list of Algorithms and Data Structures implementa
 | [Linear Regression](https://en.wikipedia.org/wiki/Linear_regression) | | | | [:white_check_mark:](linear_regression/linear_regression.py) | | | |
 | [Linear Search](https://en.wikipedia.org/wiki/Linear_search) | [:white_check_mark:](linear_search/linear_search.c) | | [:white_check_mark:](linear_search/LinearSearch.java) | [:white_check_mark:](linear_search/linear_search.py) | [:white_check_mark:](linear_search/linear-search.go) | [:white_check_mark:](linear_search/linearSearch.js) | [:white_check_mark:](linear_search/LinearSearch.cs) |
 | [Longest Common Subsequence](http://www.geeksforgeeks.org/dynamic-programming-set-4-longest-common-subsequence) | [:white_check_mark:](longest_common_subsequence/longestCommonSubsequence.c) | | [:white_check_mark:](longest_common_subsequence/LongestCommonSubsequence.java) | [:white_check_mark:](longest_common_subsequence/longest_common_subsequence.py) | [:white_check_mark:](longest_common_subsequence/longestCommonSubsequence.go) | [:white_check_mark:](longest_common_subsequence/longestCommonSubsequence.js) | |
-| [Longest Palindromic Substring](http://www.geeksforgeeks.org/longest-palindrome-substring-set-1/) | | [:white_check_mark:](longest_palindromic_substring/longest_palindromic_substring.cpp) | [:white_check_mark:](longest_palindromic_substring/LongestPalindromicSubstring.java) | [:white_check_mark:](longest_palindromic_substring/longest_palindromic_substring.py) | | [:white_check_mark:](longest_palindromic_substring/longestPalindromicSubstring.js) | |
+| [Longest Palindromic Substring](http://www.geeksforgeeks.org/longest-palindrome-substring-set-1/) | | [:white_check_mark:](longest_palindromic_substring/longest_palindromic_substring.cpp) | [:white_check_mark:](longest_palindromic_substring/LongestPalindromicSubstring.java) | [:white_check_mark:](longest_palindromic_substring/longest_palindromic_substring.py) | [:white_check_mark:](longest_palindromic_substring/longest_palindromic_substring.go) | [:white_check_mark:](longest_palindromic_substring/longestPalindromicSubstring.js) | |
 | [Merge Sort](https://www.khanacademy.org/computing/computer-science/algorithms/merge-sort/a/overview-of-merge-sort) | [:white_check_mark:](merge_sort/merge_sort.c) | | [:white_check_mark:](merge_sort/MergeSort.java) | [:white_check_mark:](merge_sort/merge_sort.py) | [:white_check_mark:](merge_sort/merge_sort.go) | [:white_check_mark:](merge_sort/mergeSort.js) | [:white_check_mark:](merge_sort/MergeSort.cs) |
 | [Modular Exponential](http://www.geeksforgeeks.org/modular-exponentiation-power-in-modular-arithmetic/) | [:white_check_mark:](modular_exponential/modular_exponential.c) | | [:white_check_mark:](modular_exponential/ModularExponential.java) | [:white_check_mark:](modular_exponential/modular_exponential.py) | [:white_check_mark:](modular_exponential/modular_exponential.go) | [:white_check_mark:](modular_exponential/modularExponential.js) | |
 | [N-Queen Problem](https://en.wikipedia.org/wiki/Eight_queens_puzzle) | | [:white_check_mark:](n_queen_problem/NQueenProblem.cpp) | [:white_check_mark:](n_queen_problem/NQueenProblem.java) | [:white_check_mark:](n_queen_problem/n_queen_problem.py) | [:white_check_mark:](n_queen_problem/n_queen_problem.go) | | |
diff --git a/longest_palindromic_substring/longest_palindromic_substring.go b/longest_palindromic_substring/longest_palindromic_substring.go
new file mode 100644
index 00000000..df233a90
--- /dev/null
+++ b/longest_palindromic_substring/longest_palindromic_substring.go
@@ -0,0 +1,46 @@
+package main
+
+import "fmt"
+
+// expandAroundCenter returns length of palindromic substring
+func expandAroundCenter(runes []rune, left int, right int) int {
+	n := len(runes)
+
+	for left >= 0 && right < n && runes[left] == runes[right] {
+		left--
+		right++
+	}
+	return right - left - 1
+}
+
+// LongestPalindromicSubstring returns the longest palindromic substring in
+// provided string.
+// Time complexity: O(n^2)
+// Space complexity: O(1)
+func LongestPalindromicSubstring(str string) string {
+	start, end := 0, 0
+	runes := []rune(str)
+
+	for i := 0; i < len(runes); i++ {
+		a := expandAroundCenter(runes, i, i)
+		b := expandAroundCenter(runes, i, i+1)
+
+		length := 0
+		if a > b {
+			length = a
+		} else {
+			length = b
+		}
+
+		if length > end-start {
+			start = i - (length-1)/2
+			end = i + length/2
+		}
+	}
+	return string(runes[start : end+1])
+}
+
+func main() {
+	str := "And the longest palindrome is... neveroddoreven!"
+	fmt.Println(LongestPalindromicSubstring(str))
+}