139. Word Break

class Solution {
public:
    bool wordBreak(string s, vector<string>& wordDict) {
        int n = s.length();     if (n == 0) return false;
        unordered_set<string> wordSet(wordDict.begin(), wordDict.end());
        vector<bool> dp(n+1, false);
        dp[0] = true;
        for (int i = 0; i < n; i++)
            for (int j = i; j >= 0; --j)
                if (dp[j] && wordSet.count(s.substr(j, i-j+1))) {
                    dp[i+1] = true;
                    break;
                }
        return dp[n];
    }
};