Add Regular Expression Matching

Author: thanthtooaung-coding

README.md

@@ -7,7 +7,7 @@ This repository contains my solutions to various LeetCode problems. Each solutio
 
 ## LeetCode Stats
 - **Profile**: [Vinnn_96](https://leetcode.com/u/Vinnn_96/)
-- **Problems Solved**: 11
+- **Problems Solved**: 12
 - **Languages**: Java
 
 ## Repository Structure
@@ -29,6 +29,7 @@ Each problem has its own directory containing:
 | 7 | [Reverse Integer](Reverse%20Integer) | Medium | [Java](Reverse%20Integer/Solution.java) | Math |
 | 8 | [String to Integer (atoi)](./String%20to%20Integer%20(atoi)) | Medium | [Java](./String%20to%20Integer%20(atoi)/Solution.java) | String |
 | 9 | [Palindrome Number](Palindrome%20Number) | Easy | [Java](Palindrome%20Number/Solution.java) | Math |
+| 10 | [Regular Expression Matching](Regular%20Expression%20Matching) | Hard | [Java](Regular%20Expression%20Matching/Solution.java) | String, Dynamic Programming, Recursion |
 | 214 | [Shortest Palindrome](Shortest%20Palindrome) | Hard | [Java](Shortest%20Palindrome/Solution.java) | String, Rolling Hash, String Matching, Hash Function |
 | 2924 | [Find Champion II](Find%20Champion%20II) | Medium | [Java](Find%20Champion%20II/Solution.java) | Graph |
 
@@ -37,7 +38,7 @@ Each problem has its own directory containing:
 ### By Difficulty
 - Easy: 2
 - Medium: 7
-- Hard: 2
+- Hard: 3
 
 ### By Topics
 - Array Problems
@@ -52,8 +53,10 @@ Each problem has its own directory containing:
   - [Zigzag Conversion](Zigzag%20Conversion)
   - [Shortest Palindrome](Shortest%20Palindrome)
   - [String to Integer (atoi)](String%20to%20Integer%20(atoi))
+  - [Regular Expression Matching](Regular%20Expression%20Matching)
 - Dynamic Programming Problems
   - [Longest Palindromic Substring](Longest%20Palindromic%20Substring)
+  - [Regular Expression Matching](Regular%20Expression%20Matching)
 - Linked List Problems
   - [Add Two Numbers](Add%20Two%20Numbers)
 - Math Problems
@@ -66,6 +69,8 @@ Each problem has its own directory containing:
   - [Find Champion II](Find%20Champion%20II)
 - String Matching Problems
   - [Shortest Palindrome](Shortest%20Palindrome)
+- Recursion Problems
+  - [Regular Expression Matching](Regular%20Expression%20Matching)
 
 ## About LeetCode
 

Regular Expression Matching/README.md

@@ -0,0 +1,48 @@
+# Regular Expression Matching
+
+## LeetCode Problem
+- Problem Number: 10
+- Problem Link: https://leetcode.com/problems/regular-expression-matching/
+- Difficulty: Hard
+- Topics: String, Dynamic Programming, Recursion
+
+## Problem Description
+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).
+
+Note:
+- For each appearance of '*', there will be a previous valid character to match
+- The pattern must match the entire string, not just a portion of it
+
+## Examples
+
+### Example 1:
+```
+Input: s = "aa", p = "a"
+Output: false
+Explanation: "a" does not match the entire string "aa".
+```
+
+### Example 2:
+```
+Input: s = "aa", p = "a*"
+Output: true
+Explanation: '*' means zero or more of the preceding element, 'a'. Therefore, by repeating 'a' once, it becomes "aa".
+```
+
+### Example 3:
+```
+Input: s = "ab", p = ".*"
+Output: true
+Explanation: ".*" means "zero or more (*) of any character (.)".
+```
+
+## Constraints
+- 1 <= s.length <= 20
+- 1 <= p.length <= 20
+- 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.

Regular Expression Matching/Solution.java

@@ -0,0 +1,23 @@
+class Solution {
+    public boolean isMatch(String s, String p) {
+        return isMatchHelper(s, p, 0, 0, new Boolean[s.length() + 1][p.length() + 1]);
+    }
+
+    private boolean isMatchHelper(String s, String p, int i, int j, Boolean[][] memo) {
+        if (memo[i][j] != null) {
+            return memo[i][j];
+        }
+        if (j == p.length()) {
+            return memo[i][j] = (i == s.length());
+        }
+        boolean firstMatch = (i < s.length() &&
+                              (s.charAt(i) == p.charAt(j) || p.charAt(j) == '.'));
+        if (j + 1 < p.length() && p.charAt(j + 1) == '*') {
+            memo[i][j] = isMatchHelper(s, p, i, j + 2, memo) ||
+                         (firstMatch && isMatchHelper(s, p, i + 1, j, memo));
+        } else {
+            memo[i][j] = firstMatch && isMatchHelper(s, p, i + 1, j + 1, memo);
+        }
+        return memo[i][j];
+    }
+}