lc.647 回文子串+扩展

回文子串

1.dp法

class Solution {
public:
    int countSubstrings(string s) {
        int count = 0;
        vector> dp(s.size(), vector(s.size(), false));
        for(int i = s.size()-1; i >= 0; i--)
        {
            for(int j = i; j < s.size(); j++)
            {
                if(s[i] == s[j] && (j - i <= 1 || dp[i+1][j-1]))
                {
                    count++;
                    dp[i][j] = true;
                }
            }
        }
        return count;
    }
};

2.中心扩散法

class Solution {
public:
    int countSubstrings(string s) {
        int res = 0;
        for(int i = 0; i < s.size(); i++)
        {
            res += extend(s, i, i);
            res += extend(s, i, i+1);
        }
        return res;
    }

    int extend(const string s, int l, int r)
    {
        int count = 0;
        while(l >= 0 && r < s.size() && s[l] == s[r])
        {
            count++;
            l--, r++;
        }    
        return count;
    }
};

3.中心扩散并输出

class Solution {
public:

    string str;
    int countSubstrings(string s) {
        vector ans; 
        int res = 0;
        for(int i = 0; i < s.size(); i++)
        {
            str.clear();
            res += extend(s, i, i, str, ans);
            str.clear();
            res += extend(s, i, i+1, str, ans);
        }
        for(auto i : ans)
            cout << i << ' ';
        return res;
    }

    int extend(const string s, int l, int r, string &str, vector& ans)
    {
        int count = 0;
        while(l >= 0 && r < s.size() && s[l] == s[r])
        {
            str = s.substr(l, r-l+1);
            ans.push_back(str);
            count++;
            l--, r++;
        }    
        return count;
    }
};