Solution.java

class Solution {
    private String s;
    private String[][] f;

    public String encode(String s) {
        this.s = s;
        int n = s.length();
        f = new String[n][n];
        for (int i = n - 1; i >= 0; --i) {
            for (int j = i; j < n; ++j) {
                f[i][j] = g(i, j);
                if (j - i + 1 > 4) {
                    for (int k = i; k < j; ++k) {
                        String t = f[i][k] + f[k + 1][j];
                        if (f[i][j].length() > t.length()) {
                            f[i][j] = t;
                        }
                    }
                }
            }
        }
        return f[0][n - 1];
    }

    private String g(int i, int j) {
        String t = s.substring(i, j + 1);
        if (t.length() < 5) {
            return t;
        }
        int k = (t + t).indexOf(t, 1);
        if (k < t.length()) {
            int cnt = t.length() / k;
            return String.format("%d[%s]", cnt, f[i][i + k - 1]);
        }
        return t;
    }
}