No.010 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


思路：

参考自：http://www.tuicool.com/articles/Ebiymu

 1 public boolean isMatch(String s, String p) {
 2     // base case
 3     if (p.length() == 0) {
 4         return s.length() == 0;
 5     }
 6  
 7     // special case
 8     if (p.length() == 1) {
 9         // if the length of s is 0, return false
10         if (s.length() < 1) {
11             return false;
12         }
13         //if the first does not match, return false
14         else if ((p.charAt(0) != s.charAt(0)) && (p.charAt(0) != '.')) {
15             return false;
16         }
17  
18         // otherwise, compare the rest of the string of s and p.
19         else {
20             return isMatch(s.substring(1), p.substring(1));
21         }
22     }
23  
24     // case 1: when the second char of p is not '*'
25     if (p.charAt(1) != '*') {
26         if (s.length() < 1) {
27             return false;
28         }
29         if ((p.charAt(0) != s.charAt(0)) && (p.charAt(0) != '.')) {
30             return false;
31         } else {
32             return isMatch(s.substring(1), p.substring(1));
33         }
34     }
35  
36     // case 2: when the second char of p is '*', complex case.
37     else {
38         //case 2.1: a char & '*' can stand for 0 element
39         if (isMatch(s, p.substring(2))) {
40             return true;
41         }
42  
43         //case 2.2: a char & '*' can stand for 1 or more preceding element, 
44         //so try every sub string
45         int i = 0;
46         while (i<s.length() && (s.charAt(i)==p.charAt(0) || p.charAt(0)=='.')){
47             if (isMatch(s.substring(i + 1), p.substring(2))) {
48                 return true;
49             }
50             i++;
51         }
52         return false;
53     }
54 }