题目：

题解：

class Solution {
private:unordered_map<int, vector<string>> ans;unordered_set<string> wordSet;public:vector<string> wordBreak(string s, vector<string>& wordDict) {wordSet = unordered_set(wordDict.begin(), wordDict.end());backtrack(s, 0);return ans[0];}void backtrack(const string& s, int index) {if (!ans.count(index)) {if (index == s.size()) {ans[index] = {""};return;}ans[index] = {};for (int i = index + 1; i <= s.size(); ++i) {string word = s.substr(index, i - index);if (wordSet.count(word)) {backtrack(s, i);for (const string& succ: ans[i]) {ans[index].push_back(succ.empty() ? word : word + " " + succ);}}}}}
};