leetcode: TC of top-down memorization

class Solution {
    public boolean wordBreak(String s, List<String> wordDict) {
        return dfs(s,wordDict,0,new Boolean[s.length()]);
    }
    // Given n as the length of s, m as the length of wordDict, and k as the average length of the words in wordDict,
    public boolean dfs(String s, List<String> wordDict,int start,Boolean[] memo) {
        if(start == s.length()){
            return true;
        }
        if(memo[start] != null) return memo[start]; // n states
        boolean res = false;
        for(String str: wordDict){ // to calculate each state, iterate m times
            if(start+str.length() > s.length()) continue;
            String t = s.substring(start,start+str.length()); // cost k times
            if(t.equals(str) && dfs(s,wordDict,start+str.length(),memo)){
               res = true;
            }
        }
        return memo[start] = res;
    }
}