Day 57 动态规划
647. 回文子串
暴力解法
class Solution {bool isPalindromic(const string &s) {int i = 0, j = s.size() - 1;while (i < j) {if (s[i] != s[j]) {return false;}i++; j--;}return true;}public:int countSubstrings(string s) {int sum = 0;for (int i = 0; i < s.size(); i++) {for (int len = 1; i + len <= s.size(); len++) {string tmp = s.substr(i, len);if (isPalindromic(tmp)) {sum++;}}}return sum;}
};
动态规划
class Solution {
public:int countSubstrings(string s) {int len = s.size(), cnt = 0;vector<vector<bool>> dp(len, vector<bool>(len, false));for (int i = len - 1; i >= 0; i--){for (int j = i; j < len; j++){if (s[i] == s[j]){if (j - i <= 1){cnt++;dp[i][j] = true;}else if (dp[i + 1][j - 1]){cnt++;dp[i][j] = true;}}}}return cnt;}
};
双指针
从当前位置往两边找
class Solution {int check(const string &s, int left, int right){int cnt = 0;while (left >= 0 && right < s.size() && s[left] == s[right]){left--;right++;cnt++;}return cnt;}public:int countSubstrings(string s) {int cnt = 0;for (int i = 0; i < s.size(); i++){cnt += check(s, i, i);cnt += check(s, i, i + 1);}return cnt;}
};
516. 最长回文子序列
class Solution {
public:int longestPalindromeSubseq(string s) {int len = s.size(), maxLen = 1;vector<vector<int>> dp(len, vector<int>(len, 0));for (int i = 0; i < len; i++){dp[i][i] = 1;}for (int i = len - 1; i >= 0; i--){for (int j = i + 1; j < len; 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]);}if (dp[i][j] > maxLen){maxLen = dp[i][j];}}}return maxLen;}
};
)
