[动态规划] leetcode 647 Palindromic Substrings

problem:https://leetcode.com/problems/palindromic-substrings/

        对于每个字符，找到前面和它匹配的字符，再看中间夹着的字符串是不是len - 2的回文串，如果是，结果加一。

class Solution {
public:
    int countSubstrings(string s) {
        int res = s.size();

        vector<int> dp(2 * s.size() - 1);

        for (int i = 0; i < dp.size(); i++)
        {
            if ((i % 2) == 0) dp[i] = 1;
            else dp[i] = 0;            
        }
        vector<unordered_set<int>> nums(256);
        for (int i = 0; i < s.size(); i++) // end
        {
            for(auto& j : nums[s[i]])
            {
                int len = i - j + 1;
    
                if (dp[i + j] == len - 2)
                {
                    dp[i + j] = len;
                    res++;
                }        
                
            }
            nums[s[i]].insert(i);
        }
        return res;
    }
};