剑指 Offer 19. 正则表达式匹配https://leetcode.cn/problems/zheng-ze-biao-da-shi-pi-pei-lcof/

动态规划：通过dp数组剪枝

只需要对各种情况进行分类处理即可

vector<vector<int>> dp;

bool helper(const string& s, const int i, const string& p, const int j)
{
	// base case
	if (j == p.size())
	{
		return i == s.size();
	}
	if (i == s.size())
	{
		if ((p.size() - j) % 2 == 1)
		{
			return false;
		}

		for (int k = j + 1; k < p.size(); k += 2)
		{
			if (p[k] != '*')
			{
				return false;
			}
		}
		return true;
	}

	if (dp[i][j] != -1)
	{
		return dp[i][j];
	}

	bool res = false;
	if (s[i] == p[j] || p[j] == '.')
	{
		if (j + 1 < p.size() && p[j + 1] == '*')
		{
			res = helper(s, i, p, j + 2) || helper(s, i + 1, p, j);
		}
		else
		{
			res = helper(s, i + 1, p, j + 1);
		}
	}
	else
	{
		if (j + 1 < p.size() && p[j + 1] == '*')
		{
			res = helper(s, i, p, j + 2);
		}
	}

	dp[i][j] = res;
	return res;
}

bool isMatch(string s, string p) {

	int i = s.size(), j = p.size();

	dp.resize(i, vector<int>(j, -1));

	return helper(s, 0, p, 0);
}