My Java Solution using hashMap and backtracking 163ms

  • 10

    The idea is borrowed from the discussion( , which is to first calculating all possible prefix, then do backtracking.

    We can use Trie or hashMap to store the prefix information, while I think Trie might be more hard to implement, without saving any space. So I use hashMap to store prefix information.

    public class Solution {
        public List<List<String>> wordSquares(String[] words) {
            List<List<String>> ret = new ArrayList<List<String>>();
            if(words.length==0 || words[0].length()==0) return ret;
            Map<String, Set<String>> map = new HashMap<>();
            int squareLen = words[0].length();
            // create all prefix
            for(int i=0;i<words.length;i++){
                for(int j=-1;j<words[0].length();j++){
                    if(!map.containsKey(words[i].substring(0, j+1))) map.put(words[i].substring(0, j+1), new HashSet<String>());
                    map.get(words[i].substring(0, j+1)).add(words[i]);
            helper(ret, new ArrayList<String>(), 0, squareLen, map);
            return ret;
        public void helper(List<List<String>> ret, List<String> cur, int matched, int total, Map<String, Set<String>> map){
            if(matched == total) {ret.add(new ArrayList<String>(cur));return;}
            // build search string
            StringBuilder sb = new StringBuilder();
            for(int i=0;i<=matched-1;i++) sb.append(cur.get(i).charAt(matched));
            // bachtracking
            Set<String> cand = map.get(sb.toString());
            if(cand==null) return;
            for(String str:cand){
                helper(ret, cur, matched+1, total, map);

  • 1


  • 0

    @YuTingLiu Thank you so much for your sharing. All of your solutions are really helpful. Thank you.

  • 0

    You can save some space by change the map to Map<String, Set<Integer>>, map value is index list

  • 0

    after changing the last line '''cur.remove(cur.size()-1);'''
    i cannot pass some test. Could anyone explain to me why?

    Thank you!

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