[LeetCode] 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

很费时间的一道题。注意这里的*不是一般通配符里的*，而是表示的前一个字符可以重复0次到多次。*因此在这个题目里也不会以打头形式出现。

这题需要处理的不同情况非常的多。。。

	public boolean isMatch(String s, String p) {
		if (p.length() == 0)
			return s.length() == 0;

		// strict match
		if (p.length() == 1 || p.charAt(1) != '*') {
			// note '*' won't be a beginning character
			if (s.length() < 1
					|| (p.charAt(0) != '.' && s.charAt(0) != p.charAt(0)))
				return false;
			else
				return isMatch(s.substring(1), p.substring(1));
		}
		// need to match one character followed by a '*'
		else {
			// k must start from -1
			// otherwise, the check will fail if s is an empty string
			for (int k = -1; k < s.length()
			// empty string || match
					&& (k < 0 || p.charAt(0) == '.' || s.charAt(k) == p
							.charAt(0)); k++) {
				if (isMatch(s.substring(k + 1), p.substring(2)))
					return true;
			}

			return false;
		}
	}