feat: add Longest Palindrome Substring solution (#1210)

* feat: add Longest Palindrome Substring solution

* fix: update formatting and allocate new results string

* fix: update formatting, fix bug related to the string copy

* fix: add parantheses for one line if statement

* fix: add comments for library inclusions

leetcode/DIRECTORY.md

@@ -9,6 +9,7 @@
 |    2 | [Add Two Numbers](https://leetcode.com/problems/add-two-numbers)                                                                                       | [C](./src/2.c)    | Medium     |
 |    3 | [Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters)                         | [C](./src/3.c)    | Medium     |
 |    4 | [Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays)                                                               | [C](./src/4.c)    | Hard       |
+|    5 | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring)                                                           | [C](./src/5.c)    | Medium     |
 |    6 | [Zigzag Conversion](https://leetcode.com/problems/zigzag-conversion)                                                                                   | [C](./src/6.c)    | Medium     |
 |    7 | [Reverse Integer](https://leetcode.com/problems/reverse-integer)                                                                                       | [C](./src/7.c)    | Medium     |
 |    8 | [String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi)                                                                       | [C](./src/8.c)    | Medium     |

leetcode/src/5.c

@@ -0,0 +1,52 @@
+/**
+ * Find longest palindrome by traversing the string and checking how
+ * long palindrome can be constructed from each element by going left and right.
+ * Checking palindromes of types '..aa..' and '..bab..'
+ */
+
+#include <stdlib.h> /// for allocating new string via malloc()
+#include <string.h> /// for copying the contents of the string via strncpy()
+
+char * longestPalindrome(char * s) {
+    int si_max = 0, ei_max = 0, sz_max = 0, sz, i, delta_i;
+    char ch, *s_longest;
+    if (s[1] == '\0') return s;
+
+    for (ch = s[1], i = 1; ch != '\0'; ch = s[++i]) {
+        if (s[i - 1] == ch) {
+            sz = 2;
+            delta_i = 1;
+            while (i - 1 - delta_i >= 0 && s[i + delta_i] != '\0' && s[i - 1 - delta_i] == s[i + delta_i]) {
+                sz += 2;
+                delta_i += 1;
+            }
+            if (sz > sz_max) {
+                sz_max = sz;
+                si_max = i - 1 - delta_i + 1;
+                ei_max = i + delta_i - 1;
+            }
+        }
+    }
+
+    for (ch = s[0], i = 1; ch != '\0'; ch = s[++i]) {
+        sz = 1;
+        delta_i = 1;
+        while (i - delta_i >= 0 && s[i + delta_i] != '\0' && s[i - delta_i] == s[i + delta_i]) {
+            sz += 2;
+            delta_i += 1;
+        }
+        if (sz > sz_max) {
+            sz_max = sz;
+            si_max = i - delta_i + 1;
+            ei_max = i + delta_i - 1;
+        }
+    }
+
+    if ((s_longest = (char *) malloc(sizeof(s))) == NULL) {
+        return NULL;
+    }
+    strncpy(s_longest, s + si_max, sz_max);
+    s_longest[sz_max] = '\0';
+
+    return s_longest;
+}