算法训练营第五十七天|647. 回文子串 516.最长回文子序列

Leetcode647. 回文子串

文章链接：代码随想录
题目链接：647. 回文子串

思路：动态规划

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

双指针：

class Solution {
public:int countSubstrings(string s) {int res = 0;for (int i = 0; i < s.size(); i++){res += isSub(i, i, s.size(), s);res += isSub(i, i + 1, s.size(), s);}return res;}int isSub(int i, int j, int end, string& s){int res_tmp = 0;while (i >= 0 && j < end){if (s[i] == s[j]){res_tmp++;i--;j++;}else break;}return res_tmp;}
};

Leetcode516.最长回文子序列

文章链接：代码随想录
题目链接：516.最长回文子序列

思路：dp[i + 1][j - 1] + 2 这里注意相等长度是加2

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