10. Regular Expression Matching && 44. Wildcard Matching

10. Regular Expression Matching

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) {
        return isMatch(s, 0, p, 0);
    }

    private boolean isMatch(String s, int si, String p, int pi) {
        if(pi == p.length())
            return si == s.length(); 

        char pCurrent = p.charAt(pi);
        if(pi+1<p.length() && p.charAt(pi+1) == ‘*‘) //if followed by star
        {
            if(isMatch(s, si, p, pi+2)) //skip the first star part.
                return true;
            if(si<s.length() && (s.charAt(si) == pCurrent || pCurrent == ‘.‘) && isMatch(s, si+1, p, pi))
                return true;
            return false;
        }
        else //match single character
        {
            if(si == s.length())
                return false; //if s is already at the end.
            if(pCurrent != s.charAt(si) && pCurrent != ‘.‘)
                return false; //cannot find the match in s.
            return isMatch(s, si+1, p, pi+1);
        }
    }
}