taskG 03112013 GP of Siberia

    int n;
    String[] dict;
    boolean[][] isChecked;

    private int getPrefixLength(String a, int posA, String b, int posB) {
        int result = 0;
        while (result + posA < a.length() && result + posB < b.length()
        && a.charAt(result + posA) == b.charAt(result + posB)) {
            result++;
        }
        return result;
    }

    private boolean slv(String str, int pos, int ind) {
        boolean result = false;
        if (isChecked[ind][pos]) {
            return false;
        }
        isChecked[ind][pos] = true;
        int commonPrefix;
        for (int i = 0; i < dict.length && !result; i++) {
            commonPrefix = getPrefixLength(str, pos, dict[i], 0);
            if (commonPrefix != Math.min(str.length() - pos, dict[i].length())) {
                continue;
            }
            if (pos + commonPrefix == str.length() && commonPrefix == dict[i].length()) {
                return true;
            }
            if (pos + commonPrefix == str.length()) {
                if (!isChecked[i][commonPrefix]) {
                    result |= slv(dict[i], commonPrefix, i);
                }
            } else {
                if (!isChecked[ind][pos + commonPrefix]) {
                    result |= slv(str, pos + commonPrefix, ind);
                }
            }
        }
        return result;
    }

    public void solve() throws NumberFormatException, IOException {
        n = nextInt();
        dict = new String[n];
        isChecked = new boolean[n][];
        for (int i = 0; i < dict.length; i++) {
            dict[i] = in.readLine();
            isChecked[i] = new boolean[dict[i].length()];
            for (int j = 0; j < dict[i].length(); j++) {
                isChecked[i][j] = false;
            }
        }
        for (int i = 0; i < n - 1; i++) {
            for (int j = i + 1; j < n; j++) {
                int ind = i;
                String a = dict[i];
                String b = dict[j];
                int com = getPrefixLength(a, 0, b, 0);
                if (com != Math.min(a.length(), b.length())) {
                    continue;
                }
                if (b.length() > a.length()) {
                    a = b;
                    ind = j;
                }
                if (isChecked[ind][com]) {
                    continue;
                }
                if (slv(a, com, ind)) {
                    out.println("YES");
                    return;
                }
            }
        }
        out.println("NO");
    }