139. 单词拆分

Posted 2022-08-26 lasclocker

• 参考: https://leetcode-cn.com/problems/word-break/solution/dan-ci-chai-fen-by-leetcode/

• 暴力递归 超时，时间复杂度:O(N^N), 空间复杂度O(N): 递归的深度

    public boolean wordBreak(String s, List<String> wordDict) 
        return wordBreakSet(s, new HashSet<>(wordDict));
    

    private boolean wordBreakSet(String s, HashSet<String> wordDict) 
        if (s.isEmpty()) 
            return true;
        
        for (int i = 0; i < s.length(); i++) 
            String before = s.substring(0, i);
            String after = s.substring(i);
            if (wordDict.contains(after) && wordBreakSet(before, wordDict)) 
                return true;
            
        
        return false;
    

• 递归+记忆 accept，时间复杂度:O(N^2), 空间复杂度O(N):递归的深度

public boolean wordBreak(String s, List<String> wordDict) 
        return wordBreakSet(s, new HashSet<>(wordDict), new HashSet<>());
    

    private boolean wordBreakSet(String s, HashSet<String> wordDict, HashSet<String> memory) 
        if (s.isEmpty()) 
            return true;
        
        if (memory.contains(s)) return false; //记忆
        for (int i = 0; i < s.length(); i++) 
            String before = s.substring(0, i);
            String after = s.substring(i);
            if (wordDict.contains(after) && wordBreakSet(before, wordDict, memory)) 
                return true;
            
        
        memory.add(s); //记忆
        return false;
    

• 单纯的BFS，超时

public boolean wordBreak(String s, List<String> wordDict) 
        LinkedList<String> queue = new LinkedList<>();
        queue.add(s);
        while (!queue.isEmpty()) 
            String seg = queue.poll();
            for (String word : wordDict) 
                if (seg.startsWith(word)) 
                    String otherSeg = seg.substring(word.length());
                    if (otherSeg.isEmpty()) 
                        return true;
                    
                    queue.add(otherSeg);
                
            
        
        return false;
    

• BFS + 记忆，accept

public boolean wordBreak(String s, List<String> wordDict) 
        LinkedList<String> queue = new LinkedList<>();
        queue.add(s);
        Set<String> memory = new HashSet<>(); //记忆
        while (!queue.isEmpty()) 
            String seg = queue.poll();
            if (!memory.contains(seg))  //记忆
                for (String word : wordDict) 
                    if (seg.startsWith(word)) 
                        String otherSeg = seg.substring(word.length());
                        if (otherSeg.isEmpty()) 
                            return true;
                        
                        queue.add(otherSeg);
                    
                
                memory.add(seg); //记忆
            
        
        return false;