From 68bdbfb0a58560041f2fd7313b87ccf5ebd28777 Mon Sep 17 00:00:00 2001
From: Yashvardhan Singh <68675629+PythonicBoat@users.noreply.github.com>
Date: Thu, 10 Nov 2022 06:54:15 +0530
Subject: [PATCH] feat: add LeetCode problem 10 (#1033)

* updated readme

* Create 10.c

* Update README.md

* added documentation

* chore: apply suggestions from code review

* Update DIRECTORY.md

Co-authored-by: David Leal <halfpacho@gmail.com>
---
 leetcode/DIRECTORY.md |  1 +
 leetcode/src/10.c     | 59 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 leetcode/src/10.c

diff --git a/leetcode/DIRECTORY.md b/leetcode/DIRECTORY.md
index 0de9c220..1283f175 100644
--- a/leetcode/DIRECTORY.md
+++ b/leetcode/DIRECTORY.md
@@ -12,6 +12,7 @@
 | 7    | [Reverse Integer](https://leetcode.com/problems/reverse-integer/)                                                               | [C](./src/7.c)    | Easy       |
 | 8    | [String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi)                                                | [C](./src/8.c)    | Medium     |
 | 9    | [Palindrome Number](https://leetcode.com/problems/palindrome-number/)                                                           | [C](./src/9.c)    | Easy       |
+| 10   | [Regular Expression Matching](https://leetcode.com/problems/regular-expression-matching/)                                       | [C](./src/10.c)   | Hard       |
 | 11   | [Container With Most Water](https://leetcode.com/problems/container-with-most-water/)                                           | [C](./src/11.c)   | Medium     |
 | 12   | [Integer to Roman](https://leetcode.com/problems/integer-to-roman)                                                              | [C](./src/12.c)   | Medium     |
 | 13   | [Roman to Integer](https://leetcode.com/problems/roman-to-integer/)                                                             | [C](./src/13.c)   | Easy       |
diff --git a/leetcode/src/10.c b/leetcode/src/10.c
new file mode 100644
index 00000000..26ed6a3b
--- /dev/null
+++ b/leetcode/src/10.c
@@ -0,0 +1,59 @@
+/*
+Prompt:
+
+Given an input string s and a pattern p, implement regular expression matching with support for '.' and '*' where:
+- '.' Matches any single character.
+- '*' Matches zero or more of the preceding element.
+The matching should cover the entire input string (not partial).
+
+Constraints:
+
+1 <= s.length <= 20
+1 <= p.length <= 30
+s contains only lowercase English letters.
+p contains only lowercase English letters, '.', and '*'.
+
+It is guaranteed for each appearance of the character '*', there will be a previous valid character to match.
+*/
+
+bool isMatch(char* s, char* p);
+bool matchStar(char ch, char* s, char* p);
+
+/*
+Uses Rob pikes Regexp matcher - https://www.cs.princeton.edu/courses/archive/spr09/cos333/beautiful.html
+Implementation:
+    // match: search for regexp anywhere in text 
+    int match(char *regexp, char *text)
+    {
+        if (regexp[0] == '^')
+            return matchhere(regexp+1, text);
+        do {  
+            if (matchhere(regexp, text))
+                return 1;
+        } while (*text++ != '\0');
+        return 0;
+    }
+*/
+
+bool matchStar(char ch, char* s, char* p) {
+  do {
+    if (isMatch(s, p))
+      return true;
+  } while (*s != '\0' && (*s++ == ch || ch == '.'));
+
+  return false;
+}
+
+bool isMatch(char* s, char* p) {
+  if (*p == '\0')
+    return *s == '\0';
+
+  if (p[1] == '*')
+    return matchStar(p[0], s, p + 2);
+
+  if (*s != '\0' && (p[0] == '.' || *p == *s)) {
+    return isMatch(s + 1, p + 1);
+  }
+
+  return false;
+}