647. 回文子串

class Solution {
    public int countSubstrings(String s) {
        int len = s.length();
        int res=0;
        //如dp[i][j]存储i~j是否是回文字符串
        boolean [][] dp=new boolean[len][len];

        for (int j = 0; j < len; j++) {
            for (int i = 0; i <= j; i++) {
                if (s.charAt(i)==s.charAt(j)&&(j-i<2||dp[i+1][j-1])){
                    dp[i][j]=true;
                    res++;
                }
            }
        }
        return res;
    }
}