Word Break II

public class Solution {
    public ArrayList<String> wordBreak(String s, Set<String> dict) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<String> result = new ArrayList<String>();
        if (s == null || dict.size() <= 0) {
            return result;
        }
        int length = s.length();
        // seg(i, j) means substring t start from i and length is j can be
        // segmented into
        // dictionary words
        boolean[][] seg = new boolean[length][length + 1];
        for (int len = 1; len <= length; len++) {
            for (int i = 0; i < length - len + 1; i++) {
                String t = s.substring(i, i + len);
                if (dict.contains(t)) {
                    seg[i][len] = true;
                    continue;
                }
                for (int k = 1; k < len; k++) {
                    if (seg[i][k] && seg[i + k][len - k]) {
                        seg[i][len] = true;
                        break;
                    }
                }
            }
        }
        if (!seg[0][length]) {
            return result;
        }

        int depth = 0;
        dfs(s, seg, 0, length, depth, result, new StringBuffer(), dict);

        return result;
    }

    private static void dfs(String s, boolean[][] seg, int start, int length,
            int depth, ArrayList<String> result, StringBuffer sb, Set<String> dict) {
        if (depth == length) {
            String t = sb.toString();
            result.add(t.substring(0, t.length() - 1));
            return;
        }

        for (int len = 1; len <= length; len++) {
            if (seg[start][len]) {
                String t = s.substring(start, start + len);
                if(!dict.contains(t)){
                    continue;
                }
                int beforeAddLen = sb.length();
                sb.append(t).append(" ");
                dfs(s, seg, start + len, length, start + len, result, sb, dict);
                sb.delete(beforeAddLen, sb.length());
            }
        }
    }
}