LeetCode | Regular Expression Matching

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

思路是分别处理a*、.*、.、a的情况。

三周前写的lengthy code如下：

class Solution {
public:
    bool isMatch(const char *s, const char *p) {
        int sn = strlen(s);
        int pn = strlen(p);
        return recursive(s, sn, p, pn);
    }
    
    bool recursive(const char* s, int sn, const char* p, int pn) {
        if (sn == 0 && pn == 0) return true;
        if (pn == 0) return false;
        
        if (*(p + 1) != '') {
            if (*(p + 1) == '*') {
                if (*p == '.') { // .*
                    int n = 0;
                    while (n <= sn) {
                        if (recursive(s + n, sn - n, p + 2, pn - 2)) return true;
                        n++;
                    }
                } else { // a*
                    int n = 0;
                    while (n <= sn && *(s + n) == *p) {
                        if (recursive(s + n, sn - n, p + 2, pn - 2)) return true;
                        n++;
                    }
                    if (recursive(s + n, sn - n, p + 2, pn - 2)) return true;
                }
            } else {
                if (*p != '.' && *s != *p) return false;
                if (recursive(s + 1, sn - 1, p + 1, pn - 1)) return true;
            }
        } else {
            if (*p != '.' && *s != *p) return false;
            if (recursive(s + 1, sn - 1, p + 1, pn - 1)) return true;
        }
    }
};

今天看了Leetcode上1337的代码真是羞愧啊。http://leetcode.com/2011/09/regular-expression-matching.html

重写了一遍。思路还是一样。

class Solution {
public:
    bool isMatch(const char *s, const char *p) {
        if (*p == '') return (*s == '');
        // match single '', '.', 'a'...
        if (*(p + 1) != '*') { 
            return ((*s == *p || (*p == '.' && *s != '')) && isMatch(s + 1, p + 1));
        }
        
        // match a*, .*
        while ((*s == *p || (*p == '.' && *s != ''))) {
            if (isMatch(s++, p + 2)) return true;
        }
        
        // ignore a*， *p != '.' && *s != *p
        return isMatch(s, p + 2);
    }
};