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
这一题麻烦的地方在于回溯， 如果查到某个是*，则要往前退。

public class Solution {
    public boolean isMatch(String s, String p) {
        // Start typing your Java solution below
        // DO NOT write main() function
        assert(p!=null && (p.length()==0 || p.charAt(0)!='*'));
        if(p.length()==0) return s.length()==0;
        if(p.length()==1 || p.charAt(1)!='*'){
            if(s.length()<1 || (p.charAt(0)!='.' && p.charAt(0)!=s.charAt(0))) return false;
            return isMatch(s.substring(1),p.substring(1));
        }
        else{
            int i=-1;
            while(i<s.length() && (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;
        }
    }
}