代码随想录day57

516最长回文子序列

class Solution {
public:
    int longestPalindromeSubseq(string s) 
    {
        vector<vector<int>>dp(s.size(),vector<int>(s.size(),0));
        for(int i=0;i<s.size();i++)dp[i][i]=1;
        for(int i=s.size()-1;i>=0;i--)
        {
            for(int j=i+1;j<s.size();j++)
            {
                if(s[i]==s[j]){
                    dp[i][j]=dp[i+1][j-1]+2;
                }
                else
                {
                    dp[i][j]=max(dp[i+1][j],dp[i][j-1]);
                }
            }
        }
        return dp[0][s.size()-1];
    }
};

647回文子串

两种做法 

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

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

5最长回文子串

class Solution {
public:
    string longestPalindrome(string s) 
    {
        if(s.size()==1) return s;
        int maxlen=0;
        int index=0;
        for(int i=1;i<s.size();i++)
        {
            for(int j=0;j<=1;j++)
            {
                int slow=i-1;
                int fast=i+j;
                while(slow>=0&&fast<s.size()&&s[slow]==s[fast])
                {
                    slow--;
                    fast++;
                }
                if(maxlen<fast-slow-1)
                {
                    index=slow;
                    maxlen=fast-slow-1;
                }
            }

        }
        return s.substr(index+1,maxlen);
    }
};