最长回文子串

动态规划`

    class Solution {
public:
    string longestPalindrome(string s) {
        vector<vector<int>> dp;
        vector<int> one(s.length(),0);
        for(int i=0;i<s.length();i++)
        {
            dp.push_back(one);
        }
        for(int i=0;i<dp.size();i++)
        {
            dp[i][i]=1;
        }
        int step=1;
        int max=1;
        int kaishi=0;
        while(step<s.length())
        {
            for(int i=0;i+step<s.length();i++)
            {
                if(s[i]==s[i+step])
                {
                    if(step==1)
                        dp[i][i+step]=2;
                    else
                    {
                        if(dp[i+1][i+step-1]==step-1)
                            dp[i][i+step]=dp[i+1][i+step-1]+2;
                        else
                            dp[i][i+step]=dp[i+1][i+step-1];      
                    }
                    if(dp[i][i+step]>max)
                    {
                        max=dp[i][i+step];
                        kaishi=i;
                    }
                }
                else
                {
                    if(step==1)
                        dp[i][i+step]=1;
                    else
                        dp[i][i+step]=dp[i+1][i+step-1];
                    if(dp[i][i+step]>max)
                    {
                        max=dp[i][i+step];
                        kaishi=i;
                    }
                }
            }
            step++;
        }
        string res(max,0);
        int dd=0;
        for(int i=kaishi;i<kaishi+max;i++,dd++)
            res[dd]=s[i];
        return res;
    }
};