題目：

http://www.1point3acres.com/bbs/thread-131978-1-1.html

實現：

import java.util.*;

/**
 * Created by Min on 10/2/2017.
 */
public class LongestChain {
    private int getLongestChain(String[] words) {
        Map<Integer, Set<String>> map = new HashMap<Integer, Set<String>>();
        
 
for (String word : words) {
            Set<String> set = map.get(word.length());
            if (set == null) {
                set = new HashSet<String>();
                map.put(word.length(), set);
            }
            set.add(word);
        }
        int ans = 0;
        List<Integer> lengthList = new
 
 ArrayList<Integer>(map.keySet());
        Collections.sort(lengthList, Collections.reverseOrder());
        return helper(0, lengthList, map, "");
    }
    private int helper(int start, List<Integer> list, Map<Integer, Set<String>> map, String prev) {
        if (start == list.size()) {
            
 
return 0;
        }
        int ans = 0;
        if (start == 0) {
            for (String word : map.get(list.get(0))) {
                ans = Math.max(ans, helper(start + 1, list, map, word) + 1);
            }
        } else if (prev.length() -1 == list.get(start)) {
            Set<String> wordSet = map.get(list.get(start));
            for (int i = 0; i < prev.length(); i++) {
                String newWord = prev.substring(0, i) + prev.substring(i + 1);
                if (wordSet.contains(newWord)) {
                    wordSet.remove(newWord);
                    ans = Math.max(ans, helper(start + 1, list, map, newWord) + 1);
                }
            }
        }
        return ans;
    }
    public static void main(String[] args) {
        String[] input = {"a","ba","bca","bda","bdca"};
        LongestChain solution = new LongestChain();
        System.out.println(solution.getLongestChain(input));
    }

}

import java.util.*;

/**
 * Created by Min on 10/2/2017.
 */
public class LongestChain2 {
    public int getLongestChain(String[] words) {
        Set<String> set = new HashSet<String>();
        for (String word : words) {
            set.add(word);
        }
        HashMap<String, Integer> map = new HashMap<String, Integer>();
        int ans = 0;
        for (String word : words) {
            Integer length =map.get(word);
            if (length == null) {
                length = dfs(word, map, set);
            }
            ans = Math.max(ans, length);
        }
        return ans;
    }

    private int dfs(String word, Map<String, Integer> map, Set<String> set) {
        Integer ans = map.get(word);
        if (ans != null) {
            return ans.intValue();
        }
        ans = 0;
        for (int i = 0; i < word.length(); i++) {
            String newWord = word.substring(0, i) + word.substring(i + 1);
            ans = Math.max(ans, dfs(newWord, map, set) + 1);
        }
        map.put(word, ans);
        return ans.intValue();
    }
    public static void main(String[] args) {
        String[] input = {"a","ba","bca","bda","bdca"};
        LongestChain2 solution = new LongestChain2();
        System.out.println(solution.getLongestChain(input));
    }
}

[Two Sigma OA] Longest Chain