leetcode10.正则表达式匹配

class Solution {
public:
    bool isMatch(string s, string p) {
        int n1 = s.size(), n2 = p.size();
        vector<vector<bool>> dp(n1+1, vector<bool>(n2+1, false));
        dp[0][0] = true;
        for (int j = 2; j <= n2; j++) {
            if (p[j - 1] == '*') dp[0][j] = dp[0][j - 2];
        }
        for(int i = 1; i <= n1; ++i){
            for(int j = 1; j <= n2; ++j){
                if(s[i-1]==p[j-1]||p[j-1]=='.'){
                    dp[i][j] = dp[i-1][j-1];
                }else if(p[j-1]=='*'){
                    if( s[i-1]==p[j-2]||p[j-2]=='.'){
                        //把p[j-2]匹配0、1、多次
                        dp[i][j] = dp[i-1][j];//多次就是相当于不用*，但抵消了一个i
                        if(j>1)
                            dp[i][j] = dp[i][j]||dp[i][j-2]||dp[i-1][j-2];
                    }else if(j>1){//把p[j-2]匹配0次，相当于前移2个
                        dp[i][j] = dp[i][j-2];
                    }
                }
            }
        }
        return dp[n1][n2];

    }
};