Regular Expression Matching.java

/*
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).

The function prototype should be:
bool isMatch(const char *s, const char *p)

Some examples:
isMatch("aa","a") → false
isMatch("aa","aa") → true
isMatch("aaa","aa") → false
isMatch("aa", "a*") → true
isMatch("aa", ".*") → true
isMatch("ab", ".*") → true
isMatch("aab", "c*a*b") → true
*/

public class Solution {
    public boolean isMatch(String s, String p) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if (s == null) return p == null;
        return helper(s, p, 0, 0);
    }
    
    public boolean helper(String s, String p, int i, int j) {
        if (j == p.length()) return i == s.length();
        
        if (j == p.length() - 1 || p.charAt(j + 1) != '*') {
            if (i == s.length()) return false;
            return (p.charAt(j) == '.' || s.charAt(i) == p.charAt(j)) && helper(s, p, ++i, ++j);
        }
        
        while (i < s.length() && (p.charAt(j) == '.' || p.charAt(j) == s.charAt(i))) {
            if (helper(s, p, i++, j + 2)) return true;
        }
        return helper(s, p, i, j + 2);
    }
}