Regular Expression Matching

'.'

'*'

'.' 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;
}
}
}