题解 | 正则表达式匹配

class Solution {
  public:
    bool match(string str, string pattern) {
        int n = (int)str.size(), m = (int)pattern.size();
        vector<vector<bool>> dp(n + 1, vector<bool>(m + 1, false));
        dp[0][0] = true;

        // 空串匹配：形如 a*, a*b*, ...
        for (int j = 2; j <= m; ++j) {
            if (pattern[j - 1] == '*') dp[0][j] = dp[0][j - 2];
        }

        auto same = [&](char s, char p) {
            return p == '.' || p == s;
        };

        for (int i = 1; i <= n; ++i) {
            for (int j = 1; j <= m; ++j) {
                if (pattern[j - 1] != '*') {
                    if (same(str[i - 1], pattern[j - 1]))
                        dp[i][j] = dp[i - 1][j - 1];
                } else { // '*'
                    // 0 次
                    dp[i][j] = dp[i][j - 2];
                    // ≥1 次
                    if (same(str[i - 1], pattern[j - 2]))
                        dp[i][j] = dp[i][j] || dp[i - 1][j];
                }
            }
        }
        return dp[n][m];
    }
};