动态规划

647. 回文子串

class Solution {
    public int countSubstrings(String s) {
        char[] chars = s.toCharArray();
        int len = chars.length;
        boolean[][] dp = new boolean[len][len];
        int result = 0;
        for (int i = len - 1; i >= 0; i--) {
            for (int j = i; j < len; j++) {
                if (chars[i] == chars[j]) {
                    if (j - i <= 1) { // 情况一 和 情况二
                        result++;
                        dp[i][j] = true;
                    } else if (dp[i + 1][j - 1]) { //情况三
                        result++;
                        dp[i][j] = true;
                    }
                }
            }
        }
        return result;
    }
}

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(dp[i+1][j],dp[i][j-1]);
                    
                }
            }
        }
        return dp[0][len-1];
    }
}