10. 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

做出来了 不过代码很长 因为特殊情况很多 

solution里面的recursion的解法还好理解一点 加上了注释

class Solution {
    public boolean isMatch(String text, String pattern) {
        //匹配第一个字母
        if (pattern.isEmpty()) return text.isEmpty();
        boolean first_match = (!text.isEmpty() && 
                               (pattern.charAt(0) == text.charAt(0) || pattern.charAt(0) == '.'));
        
        if (pattern.length() >= 2 && pattern.charAt(1) == '*’){
            //1.忽略pattern中*及前面的字母
            return (isMatch(text, pattern.substring(2)) ||    
                    //2.使用*匹配 比如aaac a*c 之后将对比aac和a*c 也就是拿pattern中的a一直去匹配
                    (first_match && isMatch(text.substring(1), pattern)));    
        } else {
            //没有* 正常匹配
            return first_match && isMatch(text.substring(1), pattern.substring(1));    
        }
    }
}