10-Regular-Expression-Matching.java

class Solution {

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

        return dfs(cache, s, p, 0, 0);
    }

    private boolean dfs(boolean[][] cache, String s, String p, int i, int j) {
        if (cache[i][j] != false) return cache[i][j];

        if (i >= s.length() && j >= p.length()) return true;

        if (j >= p.length()) return false;

        boolean match =
            i < s.length() &&
            (s.charAt(i) == p.charAt(j) || p.charAt(j) == '.');

        if (j + 1 < p.length() && p.charAt(j + 1) == '*') {
            cache[i][j] =
                dfs(cache, s, p, i, j + 2) ||
                (match && dfs(cache, s, p, i + 1, j));
        } else {
            cache[i][j] = match && dfs(cache, s, p, i + 1, j + 1);
        }

        return cache[i][j];
    }
}