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") → falseisMatch("aa","aa") → trueisMatch("aaa","aa") → falseisMatch("aa", "a*") → trueisMatch("aa", ".*") → trueisMatch("ab", ".*") → trueisMatch("aab", "c*a*b") → true

解法1和2本质都一样，只不过解法2存储了局部结果，避免了重复处理一些字串，加快了计算的速度。
解法的关键思想就是提前判断下一个模式字符，从而分别处理下一个字符是‘*’以及非‘*’的情况； 需要自己看一下的是处理模式串中下一个字符是'*'的时候用的循环，用自己构造的一些输入用例来走几遍就可以理解了。

class Solution {public:    bool isMatch(const char *s, const char *p) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.          // 1        /*        if (s == NULL || p == NULL) return s == p;        if (*p == '\0') return *s == *p;        if (p[1] != '*') {            return (*p == *s || (*p == '.' && *s != '\0')) && isMatch(s + 1, p + 1);        }                int offset = -1;        do {            ++offset;            if (isMatch(s + offset, p + 2)) return true;        } while (s[offset] == *p || (*p == '.' && s[offset] != '\0'));                return false;        */                // 2        if (s == NULL || p == NULL) return s == p;        if (*p == '\0') return *s == *p;                int slen = strlen(s), plen = strlen(p);        vector<vector<int>> mflag(slen + 1, vector<int>(plen + 1, -1));        mflag[slen][plen] = 1;        for (int i = 0; i < slen; ++i) {            mflag[i][plen] = 0;        }                return isMatchHelper(s, p, mflag, 0, 0);    }    private:    bool isMatchHelper(const char *s, const char *p, vector<vector<int>> &flag, int spos, int ppos) {        if (flag[spos][ppos] >= 0) {            return flag[spos][ppos] == 1;        }                bool ret = false;        if (p[ppos + 1] != '*') {            ret = ((s[spos] == p[ppos] || (p[ppos] == '.' && s[spos] != '\0')) && isMatchHelper(s, p, flag, spos + 1, ppos + 1));            flag[spos][ppos] = (ret ? 1 : 0);            return ret;        }                int offset = -1;        do {            ++offset;            ret = isMatchHelper(s, p, flag, spos + offset, ppos + 2);            if (ret) {                flag[spos][ppos] = 1;                return ret;            }        } while (s[spos + offset] == p[ppos] || (p[ppos] == '.' && s[spos + offset] != '\0'));                flag[spos][ppos] = 0;        return false;    }    };