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。

 1 public class Solution {
 2     public boolean isMatch(String s, String p) {
 3         // Start typing your Java solution below
 4         // DO NOT write main() function
 5         assert(p!=null && (p.length()==0 || p.charAt(0)!='*'));
 6         if(p.length()==0) return s.length()==0;
 7         if(p.length()==1 || p.charAt(1)!='*'){
 8             if(s.length()<1 || (p.charAt(0)!='.' && p.charAt(0)!=s.charAt(0))) return false;
 9             return isMatch(s.substring(1),p.substring(1));
10         }
11         else{
12             int i=-1;
13             while(i<s.length() && (i<0 || p.charAt(0)=='.' || p.charAt(0)==s.charAt(i))){
14                 if(isMatch(s.substring(i+1),p.substring(2))) return true;
15                 i++;
16             }
17             return false;
18         }
19     }
20 }