Solution2.java

class Solution {
    public boolean wordBreak(String s, List<String> wordDict) {
        Trie trie = new Trie();
        for (String w : wordDict) {
            trie.insert(w);
        }
        int n = s.length();
        boolean[] f = new boolean[n + 1];
        f[n] = true;
        for (int i = n - 1; i >= 0; --i) {
            Trie node = trie;
            for (int j = i; j < n; ++j) {
                int k = s.charAt(j) - 'a';
                if (node.children[k] == null) {
                    break;
                }
                node = node.children[k];
                if (node.isEnd && f[j + 1]) {
                    f[i] = true;
                    break;
                }
            }
        }
        return f[0];
    }
}

class Trie {
    Trie[] children = new Trie[26];
    boolean isEnd = false;

    public void insert(String w) {
        Trie node = this;
        for (int i = 0; i < w.length(); ++i) {
            int j = w.charAt(i) - 'a';
            if (node.children[j] == null) {
                node.children[j] = new Trie();
            }
            node = node.children[j];
        }
        node.isEnd = true;
    }
}