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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0010_regular_expression_matching;

// #Hard #Top_100_Liked_Questions #Top_Interview_Questions #String #Dynamic_Programming #Recursion
// #Udemy_Dynamic_Programming #Big_O_Time_O(m*n)_Space_O(m*n)
// #2023_08_09_Time_1_ms_(100.00%)_Space_41.1_MB_(61.00%)

/**
 * 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 <= 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.
**/
public class Solution {
    private Boolean[][] cache;

    public boolean isMatch(String s, String p) {
        cache = new Boolean[s.length() + 1][p.length() + 1];
        return isMatch(s, p, 0, 0);
    }

    private boolean isMatch(String s, String p, int i, int j) {
        if (j == p.length()) {
            return i == s.length();
        }
        boolean result;
        if (cache[i][j] != null) {
            return cache[i][j];
        }
        boolean firstMatch = i < s.length() && (s.charAt(i) == p.charAt(j) || p.charAt(j) == '.');
        if ((j + 1) < p.length() && p.charAt(j + 1) == '*') {
            result = (firstMatch && isMatch(s, p, i + 1, j)) || isMatch(s, p, i, j + 2);
        } else {
            result = firstMatch && isMatch(s, p, i + 1, j + 1);
        }
        cache[i][j] = result;
        return result;
    }
}