Regular Expression Matching

Given an input string (s) and a pattern (p), implement regular expression matching with support for '.' and '*'.

'.' Matches any single character.
'*' Matches zero or more of the preceding element.

The matching should cover the entire input string (not partial).

Note:

• s could be empty and contains only lowercase letters a-z.

• p could be empty and contains only lowercase letters a-z, and characters like . or *.

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

class Solution {
    public boolean isMatch(String s, String p) {
        boolean dp[][] = new boolean[s.length() + 1][p.length() + 1];
        for (int i = 0; i <= s.length(); i++) {
            for (int j = 0; j <= p.length(); j++) {
                // Base cases start
                if (i == 0 && j == 0)
                    dp[i][j] = true;
                else if (j == 0)
                    dp[i][j] = false;
                else if (i == 0) {
                    if (p.charAt(j - 1) != '*')
                        dp[i][j] = false;
                    else
                        dp[i][j] = j >= 2 ? dp[i][j - 2] : false;
                } // Base cases end
                else if (s.charAt(i - 1) == p.charAt(j - 1) || p.charAt(j - 1) == '.')
                    dp[i][j] = dp[i - 1][j - 1];
                else if (p.charAt(j - 1) == '*') {
                    dp[i][j] = dp[i][j - 2];
                    if (s.charAt(i - 1) == p.charAt(j - 2) || p.charAt(j - 2) == '.')
                        dp[i][j] = dp[i][j] || dp[i - 1][j];
                } else
                    dp[i][j] = false;
            }
        }
        return dp[s.length()][p.length()];
    }
}