10. Regular Expression Matching

https://leetcode.com/problems/regular-expression-matching/description/

DP:

class Solution {
public:
    bool isMatch(string s, string p) {
        int m = s.length(), n = p.length();
        vector<vector<bool>> dp(m+1, vector<bool>(n+1, false));
        dp[0][0] = true;
        for (int j = 1; j < n; j++) {
            if (p[j] == '*')
                dp[0][j+1] = dp[0][j-1];　　// we should p[j-1]p[j]
        }
        for (int i = 1; i <= m; i++)
            for (int j = 1; j <= n; j++) {
                if (s[i-1] == p[j-1] || p[j-1] == '.')
                    dp[i][j] = dp[i-1][j-1];
                else if (j-1 > 0 && p[j-1] == '*') {
                    if (s[i-1] == p[j-2] || p[j-2] == '.')
                        dp[i][j] = dp[i][j-2] || dp[i-1][j];
                    else
                        dp[i][j] = dp[i][j-2];
                }
            }
        return dp[m][n];
    }
};

 recursion

class Solution {
public:
    bool isMatch(string s, string p) {
        int m = s.length(), n = p.length();
        if (n == 0) return m == 0;
        bool firstMatch = m > 0 && (s[0] == p[0] || p[0] == '.');
        if (n > 1 && p[1] == '*') {
            if (firstMatch && isMatch(s.substr(1), p))
                return true;
            return isMatch(s, p.substr(2));
        }
        return firstMatch && isMatch(s.substr(1), p.substr(1));
    }
};