class Solution {
public:
bool wordBreak(string s, vector& wordDict) {
for (auto word : wordDict)
m.emplace(word, true);
return wBreak(s, wordDict);
}

bool wBreak(string &s, vector& wordDict) {
if (m.count(s)) return m[s];

for (auto word : wordDict) {
string left = s.substr(0, word.size());
if (!m.count(left) || m[left] == false) continue;

string right = s.substr(word.size());
if (wBreak(right, wordDict)) return m[s]=true;
}
return m[s]=false;
}
private:
unordered_map m;
};