Solution.java

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

    void insert(String word) {
        Trie node = this;
        for (char c : word.toCharArray()) {
            c -= 'a';
            if (node.children[c] == null) {
                node.children[c] = new Trie();
            }
            node = node.children[c];
        }
        node.isEnd = true;
    }
}

class Solution {
    public int[][] indexPairs(String text, String[] words) {
        Trie trie = new Trie();
        for (String w : words) {
            trie.insert(w);
        }
        int n = text.length();
        List<int[]> ans = new ArrayList<>();
        for (int i = 0; i < n; ++i) {
            Trie node = trie;
            for (int j = i; j < n; ++j) {
                int idx = text.charAt(j) - 'a';
                if (node.children[idx] == null) {
                    break;
                }
                node = node.children[idx];
                if (node.isEnd) {
                    ans.add(new int[] {i, j});
                }
            }
        }
        return ans.toArray(new int[ans.size()][2]);
    }
}