谷歌面试题（2）

原题：

// Given a set of words (without duplicates), find all word squares you can build from them.// A sequence of words forms a valid word square if the kth row and column read the exact same string, where 0 ≤ k < max(numRows, numColumns).// For example, the word sequence ["ball","area","lead","lady"] forms a word square because each word reads the same both horizontally and vertically.// b a l l// a r e a// l e a d// l a d y// Note:    // There are at least 1 and at most 1000 words.    // All words will have the exact same length.    // Word length is at least 1 and at most 5.    // Each word contains only lowercase English alphabet a-z.

答案：

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){            cur.add(str);            helper(ret, cur, matched+1, total, map);            cur.remove(cur.size()-1);        }    }}