Regular Expression Matching (LeetCode String)

Question: 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

Idea: Brute force depth-first search.
1) If p is empty, return false;
2) compare the first character, if there is a match, continue to match the rest of characters recursively, otherwise return false;

Time: O(n^2) Space: O(n)

Code: 

 public class Solution {  
   public boolean isMatch(String s, String p) {  
     if(p.length()==0)  
       return s.length()==0;  
     if(p.length()==1||p.charAt(1)!='*')  
     {  
       if(s.length()==0||(p.charAt(0)!='.'&&s.charAt(0)!=p.charAt(0)))  
       {  
         return false;  
       }  
       return isMatch(s.substring(1),p.substring(1));  
     }  
     else  
     {  
       int len=s.length();  
       int i=-1;  
       while(i<len&&(i<0||p.charAt(0)=='.'||p.charAt(0)==s.charAt(i)))  
       {  
         if(isMatch(s.substring(i+1),p.substring(2)))  
           return true;  
         i++;  
       }  
       return false;  
     }  
   }  
 }