Solution.java

class Solution {
    private static final int MOD = (int) 1e9 + 7;

    public int countPalindromes(String s) {
        int n = s.length();
        int[][][] pre = new int[n + 2][10][10];
        int[][][] suf = new int[n + 2][10][10];
        int[] t = new int[n];
        for (int i = 0; i < n; ++i) {
            t[i] = s.charAt(i) - '0';
        }
        int[] c = new int[10];
        for (int i = 1; i <= n; ++i) {
            int v = t[i - 1];
            for (int j = 0; j < 10; ++j) {
                for (int k = 0; k < 10; ++k) {
                    pre[i][j][k] = pre[i - 1][j][k];
                }
            }
            for (int j = 0; j < 10; ++j) {
                pre[i][j][v] += c[j];
            }
            c[v]++;
        }
        c = new int[10];
        for (int i = n; i > 0; --i) {
            int v = t[i - 1];
            for (int j = 0; j < 10; ++j) {
                for (int k = 0; k < 10; ++k) {
                    suf[i][j][k] = suf[i + 1][j][k];
                }
            }
            for (int j = 0; j < 10; ++j) {
                suf[i][j][v] += c[j];
            }
            c[v]++;
        }
        long ans = 0;
        for (int i = 1; i <= n; ++i) {
            for (int j = 0; j < 10; ++j) {
                for (int k = 0; k < 10; ++k) {
                    ans += (long) pre[i - 1][j][k] * suf[i + 1][j][k];
                    ans %= MOD;
                }
            }
        }
        return (int) ans;
    }
}