feat: add longest palindromic substring algorithm

Signed-off-by: Arya Pratap Singh <[email protected]>

Author: ARYPROGRAMMER

DIRECTORY.md

@@ -112,6 +112,7 @@
   * [Longest Increasing Subsequence](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/longest_increasing_subsequence.cpp)
   * [Longest Increasing Subsequence Nlogn](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/longest_increasing_subsequence_nlogn.cpp)
   * [Longest Palindromic Subsequence](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/longest_palindromic_subsequence.cpp)
+  * [Longest Palindromic Substring](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/longest_palindromic_substring.cpp)
   * [Matrix Chain Multiplication](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/matrix_chain_multiplication.cpp)
   * [Maximum Circular Subarray](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/maximum_circular_subarray.cpp)
   * [Minimum Edit Distance](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/dynamic_programming/minimum_edit_distance.cpp)

dynamic_programming/longest_palindromic_substring.cpp

@@ -0,0 +1,341 @@
+/**
+ * @file
+ * @brief Implementation of [Longest Palindromic
+ * Substring](https://en.wikipedia.org/wiki/Longest_palindromic_substring)
+ * using dynamic programming
+ *
+ * @details
+ * The longest palindromic substring problem is to find the contiguous substring
+ * of maximum length in a given string that is also a palindrome. A
+ * [palindrome](https://en.wikipedia.org/wiki/Palindrome) is a string that
+ * reads the same forwards and backwards, such as "racecar" or "noon".
+ *
+ * This is different from the longest palindromic subsequence, where characters
+ * don't need to be contiguous. For example, for the string "banana":
+ * - Longest palindromic substring: "anana" (length 5)
+ * - Longest palindromic subsequence: "aanaa" (length 5, but not contiguous)
+ *
+ * ### Algorithm
+ * The algorithm uses dynamic programming with a 2D table where dp[i][j]
+ * represents whether the substring from index i to j is a palindrome.
+ *
+ * - All single characters are palindromes: dp[i][i] = true
+ * - For two characters: dp[i][i+1] = (s[i] == s[i+1])
+ * - For longer substrings: dp[i][j] = (s[i] == s[j]) && dp[i+1][j-1]
+ *
+ * ### Time Complexity: O(n^2) where n is the length of the input string
+ * ### Space Complexity: O(n^2) for the DP table
+ *
+ * ### Applications
+ * - DNA sequence analysis for identifying repeating patterns
+ * - Text processing and pattern recognition
+ * - Bioinformatics and genome sequencing
+ * - Data compression algorithms
+ *
+ * @author [Arya Singh](https://github.com/ARYPROGRAMMER)
+ * @see longest_palindromic_subsequence.cpp
+ */
+
+#include <cassert>   /// for assert
+#include <iostream>  /// for IO operations
+#include <string>    /// for std::string
+#include <vector>    /// for std::vector
+
+/**
+ * @namespace dynamic_programming
+ * @brief Dynamic Programming algorithms
+ */
+namespace dynamic_programming {
+/**
+ * @namespace longest_palindromic_substring
+ * @brief Functions for the [Longest Palindromic
+ * Substring](https://en.wikipedia.org/wiki/Longest_palindromic_substring)
+ * implementation
+ */
+namespace longest_palindromic_substring {
+
+/**
+ * @brief Find the longest palindromic substring in a given string
+ * @param s input string to analyze
+ * @return the longest palindromic substring found in the input
+ *
+ * @details
+ * This function uses dynamic programming to find the longest contiguous
+ * palindromic substring. It builds a table where dp[i][j] indicates whether
+ * the substring from index i to j is a palindrome.
+ *
+ * The algorithm proceeds as follows:
+ * 1. Initialize all single characters as palindromes
+ * 2. Check all substrings of length 2
+ * 3. Check longer substrings by extending from the center
+ * 4. Track the longest palindrome found
+ *
+ * ### Example
+ * Input: "babad"
+ * Output: "bab" or "aba" (both are valid)
+ *
+ * Input: "cbbd"
+ * Output: "bb"
+ */
+std::string longestPalindromicSubstring(const std::string &s) {
+    int n = s.length();
+    
+    // Edge case: empty or single character string
+    if (n <= 1) {
+        return s;
+    }
+
+    // dp[i][j] will be true if substring s[i..j] is a palindrome
+    std::vector<std::vector<bool>> dp(n, std::vector<bool>(n, false));
+
+    // Variables to track the longest palindrome
+    int maxLength = 1;  // Every single character is a palindrome
+    int start = 0;      // Starting index of longest palindrome
+
+    // All substrings of length 1 are palindromes
+    for (int i = 0; i < n; i++) {
+        dp[i][i] = true;
+    }
+
+    // Check for substrings of length 2
+    for (int i = 0; i < n - 1; i++) {
+        if (s[i] == s[i + 1]) {
+            dp[i][i + 1] = true;
+            start = i;
+            maxLength = 2;
+        }
+    }
+
+    // Check for lengths greater than 2
+    // len is the length of substring
+    for (int len = 3; len <= n; len++) {
+        // Fix the starting index
+        for (int i = 0; i < n - len + 1; i++) {
+            // Get the ending index of substring from
+            // starting index i and length len
+            int j = i + len - 1;
+
+            // Check if s[i..j] is palindrome
+            // If s[i] == s[j] and s[i+1..j-1] is a palindrome
+            if (s[i] == s[j] && dp[i + 1][j - 1]) {
+                dp[i][j] = true;
+
+                if (len > maxLength) {
+                    start = i;
+                    maxLength = len;
+                }
+            }
+        }
+    }
+
+    // Return the longest palindromic substring
+    return s.substr(start, maxLength);
+}
+
+/**
+ * @brief Alternative approach using expand around center method
+ * @param s input string to analyze
+ * @param left starting left index of potential palindrome center
+ * @param right starting right index of potential palindrome center
+ * @return length of the palindrome centered at given indices
+ *
+ * @details
+ * Helper function that expands around a center to find palindromes.
+ * This is used by the optimized O(n^2) time, O(1) space approach.
+ */
+int expandAroundCenter(const std::string &s, int left, int right) {
+    int n = s.length();
+    while (left >= 0 && right < n && s[left] == s[right]) {
+        left--;
+        right++;
+    }
+    // Return the length of the palindrome
+    return right - left - 1;
+}
+
+/**
+ * @brief Find longest palindromic substring with optimized space complexity
+ * @param s input string to analyze
+ * @return the longest palindromic substring found in the input
+ *
+ * @details
+ * This is a space-optimized version that uses the expand around center
+ * approach. It considers all possible centers (both single characters and
+ * pairs of characters) and expands outward to find palindromes.
+ *
+ * ### Time Complexity: O(n^2)
+ * ### Space Complexity: O(1) - only uses constant extra space
+ *
+ * ### Example
+ * Input: "racecar"
+ * Output: "racecar"
+ */
+std::string longestPalindromicSubstringOptimized(const std::string &s) {
+    if (s.empty()) {
+        return "";
+    }
+
+    int start = 0;
+    int maxLength = 0;
+    int n = s.length();
+
+    for (int i = 0; i < n; i++) {
+        // Check for odd length palindromes (single character center)
+        int len1 = expandAroundCenter(s, i, i);
+        // Check for even length palindromes (two character center)
+        int len2 = expandAroundCenter(s, i, i + 1);
+
+        int len = std::max(len1, len2);
+
+        if (len > maxLength) {
+            start = i - (len - 1) / 2;
+            maxLength = len;
+        }
+    }
+
+    return s.substr(start, maxLength);
+}
+
+}  // namespace longest_palindromic_substring
+}  // namespace dynamic_programming
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    // Test case 1: Basic palindrome
+    std::string test1 = "babad";
+    std::string result1 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test1);
+    std::string result1_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test1);
+    // "bab" or "aba" are both valid answers
+    assert((result1 == "bab" || result1 == "aba"));
+    assert((result1_opt == "bab" || result1_opt == "aba"));
+    std::cout << "Test 1 passed: Input \"" << test1 << "\" -> Output \""
+              << result1 << "\"" << std::endl;
+
+    // Test case 2: Even length palindrome
+    std::string test2 = "cbbd";
+    std::string result2 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test2);
+    std::string result2_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test2);
+    assert(result2 == "bb");
+    assert(result2_opt == "bb");
+    std::cout << "Test 2 passed: Input \"" << test2 << "\" -> Output \""
+              << result2 << "\"" << std::endl;
+
+    // Test case 3: Single character
+    std::string test3 = "a";
+    std::string result3 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test3);
+    std::string result3_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test3);
+    assert(result3 == "a");
+    assert(result3_opt == "a");
+    std::cout << "Test 3 passed: Input \"" << test3 << "\" -> Output \""
+              << result3 << "\"" << std::endl;
+
+    // Test case 4: Entire string is palindrome
+    std::string test4 = "racecar";
+    std::string result4 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test4);
+    std::string result4_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test4);
+    assert(result4 == "racecar");
+    assert(result4_opt == "racecar");
+    std::cout << "Test 4 passed: Input \"" << test4 << "\" -> Output \""
+              << result4 << "\"" << std::endl;
+
+    // Test case 5: No palindrome longer than 1
+    std::string test5 = "abcdef";
+    std::string result5 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test5);
+    std::string result5_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test5);
+    assert(result5.length() == 1);
+    assert(result5_opt.length() == 1);
+    std::cout << "Test 5 passed: Input \"" << test5 << "\" -> Output \""
+              << result5 << "\"" << std::endl;
+
+    // Test case 6: Complex case with multiple palindromes
+    std::string test6 = "bananas";
+    std::string result6 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test6);
+    std::string result6_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test6);
+    assert(result6 == "anana");
+    assert(result6_opt == "anana");
+    std::cout << "Test 6 passed: Input \"" << test6 << "\" -> Output \""
+              << result6 << "\"" << std::endl;
+
+    // Test case 7: All same characters
+    std::string test7 = "aaaa";
+    std::string result7 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test7);
+    std::string result7_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test7);
+    assert(result7 == "aaaa");
+    assert(result7_opt == "aaaa");
+    std::cout << "Test 7 passed: Input \"" << test7 << "\" -> Output \""
+              << result7 << "\"" << std::endl;
+
+    // Test case 8: Two character palindrome
+    std::string test8 = "ac";
+    std::string result8 =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(test8);
+    std::string result8_opt =
+        dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstringOptimized(test8);
+    assert(result8.length() == 1);
+    assert(result8_opt.length() == 1);
+    std::cout << "Test 8 passed: Input \"" << test8 << "\" -> Output \""
+              << result8 << "\"" << std::endl;
+
+    std::cout << "\nAll tests have successfully passed!" << std::endl;
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+
+    // Interactive example
+    std::cout << "\n--- Interactive Examples ---" << std::endl;
+    
+    std::vector<std::string> examples = {
+        "forgeeksskeegfor",
+        "noon",
+        "programming",
+        "abacdcaba"
+    };
+
+    for (const auto &example : examples) {
+        std::string result = dynamic_programming::longest_palindromic_substring::
+            longestPalindromicSubstring(example);
+        std::cout << "Longest palindromic substring in \"" << example
+                  << "\" is \"" << result << "\" (length: " << result.length()
+                  << ")" << std::endl;
+    }
+
+    return 0;
+}