LeetCode-in-All
10. Regular Expression Matching
Hard
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).
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 (.)”.
Example 4:
Input: s = “aab”, p = “c*a*b”
Output: true
Explanation: c can be repeated 0 times, a can be repeated 1 time. Therefore, it matches “aab”.
Example 5:
Input: s = “mississippi”, p = “mis*is*p*.”
Output: false
Constraints:
1 <= s.length <= 201 <= p.length <= 30scontains only lowercase English letters.pcontains only lowercase English letters,'.', and'*'.- It is guaranteed for each appearance of the character
'*', there will be a previous valid character to match.
Solution
class Solution {
List<List<bool?>>? cache;
bool isMatch(String s, String p) {
// Initialize the cache with null values, similar to Boolean[][] in Java
cache = List.generate(s.length + 1, (_) => List.filled(p.length + 1, null));
return isMatchHelper(s, p, 0, 0);
}
bool isMatchHelper(String s, String p, int i, int j) {
// If pattern is fully traversed, check if string is fully matched
if (j == p.length) {
return i == s.length;
}
// Return cached result if available
if (cache![i][j] != null) {
return cache![i][j]!;
}
// Check if first characters match (either the same or the pattern has '.')
bool firstMatch = i < s.length && (s[i] == p[j] || p[j] == '.');
bool result;
// Handle '*' wildcard in the pattern
if (j + 1 < p.length && p[j + 1] == '*') {
result = (firstMatch && isMatchHelper(s, p, i + 1, j)) ||
isMatchHelper(s, p, i, j + 2);
} else {
result = firstMatch && isMatchHelper(s, p, i + 1, j + 1);
}
// Store the result in the cache
cache![i][j] = result;
return result;
}
}