链接: 647. 回文子串
链接: 516.最长回文子序列
链接: 动态规划总结

647. 回文子串

理解dp数组的含义很重

class Solution {public int countSubstrings(String s) {char[] chars = s.toCharArray();boolean[][] dp = new boolean[s.length()][s.length()];int res = 0;// 遍历顺序为从下往上，从左往右，为了确保dp[i+1][j-1]首先被遍历到for(int i = s.length() - 1; i >= 0; i-- ){for(int j = i; j < s.length(); j++){if(chars[i] == chars[j]){if(j - i <= 1){// 情况一 和 情况二res++;dp[i][j] = true;}else if(dp[i+1][j-1]){ // 情况三res++;dp[i][j] = true;}}}}return res;}
}

516.最长回文子序列

class Solution {public int longestPalindromeSubseq(String s) {int len = s.length();int[][] dp = new int[len + 1][len + 1];for(int i = len -1; i >= 0; i--){dp[i][i] = 1;for(int j = i+1; j < len; j++){if(s.charAt(i) == s.charAt(j)){dp[i][j] = dp[i+1][j-1] + 2;}else{dp[i][j] = Math.max(Math.max(dp[i][j-1], dp[i+1][j]), dp[i][j]);}}}return dp[0][len - 1];}
}