[leetcode] 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

https://oj.leetcode.com/problems/regular-expression-matching/

思路1：遞歸。根據下一個字符是不是'*'分狀況判斷。

1. 若是p的下一個字符不是'*'，只需判斷當前字符是否相等，或者p[cur]='.'，遞歸處理s[1]和p[1]；
2. 若是是p的下一個'*'，則當前s和p相等或者p='.'狀況下，依次判斷s[0...s.length]和p2]是否match；

思路2：自動機？有待研究。

遞歸代碼：

public class Solution {
	public boolean isMatch(String s, String p) {
		if (s == null)
			return p == null;
		if (p == null)
			return s == null;

		int lenS = s.length();
		int lenP = p.length();

		if (lenP == 0)
			return lenS == 0;

		if (lenP == 1) {
			if (p.equals(s) || p.equals(".") && s.length() == 1) {
				return true;
			} else
				return false;
		}
		if (p.charAt(1) != '*') {
			if (s.length() > 0
					&& (p.charAt(0) == s.charAt(0) || p.charAt(0) == '.')) {
				return isMatch(s.substring(1), p.substring(1));
			}
			return false;
		} else {
			while (s.length() > 0
					&& (p.charAt(0) == s.charAt(0) || p.charAt(0) == '.')) {
				if (isMatch(s, p.substring(2)))
					return true;
				s = s.substring(1);
			}
			return isMatch(s, p.substring(2));
		}

	}

	public static void main(String[] args) {
		 System.out.println(new Solution().isMatch("aa", "a"));
		 System.out.println(new Solution().isMatch("aa", "aa"));
		 System.out.println(new Solution().isMatch("aaa", "aa"));
		 System.out.println(new Solution().isMatch("aa", "a*"));
		 System.out.println(new Solution().isMatch("aa", ".*"));
		 System.out.println(new Solution().isMatch("ab", ".*"));
		 System.out.println(new Solution().isMatch("aab", "c*a*b"));
		
		 System.out.println(new Solution().isMatch("", ""));
		 System.out.println(new Solution().isMatch("abcdeff", ".*"));
		 System.out.println(new Solution().isMatch("a", "ab*"));
		System.out.println(new Solution().isMatch("bb", ".bab"));

	}

}

參考：

http://www.programcreek.com/2012/12/leetcode-regular-expression-matching-in-java/

http://fisherlei.blogspot.com/2013/01/leetcode-wildcard-matching.html

http://leetcodenotes.wordpress.com/2013/08/25/leetcode-regular-expression-matching/