Solution.java

class Solution {
    public boolean wordBreak(String s, List<String> wordDict) {
        return check(s, wordDict, 0);
    }
    
    private HashMap<Integer, Boolean> cache = new HashMap<>();
    
    public boolean check(String s, List<String> wordDict, int index) {
        if (index == s.length()) {
            return true;
        }
        
        Boolean cachedValue = cache.get(index);
        if (cachedValue != null) {
            return cachedValue;
        }
        
        for (String word : wordDict) {
            // System.out.println("checking " + word + " on index " + index);
            if (matches(s, word, index)) {
                if (check(s, wordDict, index + word.length())) {
                    return true;
                }
            }
        }
        cache.put(index, false);
        
        return false;
    }
    
    public boolean matches(String s, String word, int index) {
        if (index + word.length() > s.length()) {
            return false;
        }
        
        for (int i = 0; i < word.length(); i++) {
            if (word.charAt(i) != s.charAt(index + i)) {
                return false;
            }
        }
        
        return true;
    }
}