题目：

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

思路：

递归的方式完成。考虑到*的情况，每次读入两位数值。

代码：

class Solution {
public:
    bool isMatch(const char *s, const char *p) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function    
       return canMatch(s, 0, p, 0);
    }
    
    bool canMatch(const char *s, int i, const char *p, int j)
    {
        if(s[i] == '\0' && p[j] == '\0')
        {
            return true;
        }
        else if(s[i] == '\0')
        {
            if(p[j+1] == '*')
            {
                return canMatch(s, i, p, j+2);
            }
            else
            {
                return false;
            }
        }
        else if(p[j] == '\0')
        {
            return false;
        }
        if(p[j+1] == '*')
        {
            if(s[i] == p[j] || p[j] == '.')
            {
                return canMatch(s, i+1, p, j) || canMatch(s, i, p, j+2);
            }
            else
            {
                return canMatch(s, i, p, j+2);
            }
        }
        else
        {
            if(s[i] == p[j])
            {
                return canMatch(s, i+1, p, j+1);
            }
            else if(p[j] == '.')
            {
                return canMatch(s, i+1, p, j+1);
            }
            else
            {
                return false;
            }
        }
    }
};