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LeetCode: Word Break II

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class Solution {
public:
vector<string> wordBreak(string s, unordered_set<string>& wordDict) {
unordered_map<string, vector<string>> map;
return foo(s, wordDict, map);
}

vector<string> foo(string s, unordered_set<string> &wordDict, unordered_map<string, vector<string>> &map)
{
if(map.find(s)!=map.end())
{
return map[s];
}

vector<string> res;
if(s.length()==0)
{
res.push_back("");
return res;
}

for(string word: wordDict)
{
if(startsWith(s, word))
{
auto subList=foo(s.substr(word.length()), wordDict, map);
for(auto sub: subList)
res.push_back(word+(sub.size()==0?"":" ")+sub);
}
}
map[s]=res;
return res;
}

bool startsWith(string lhs, string rhs)
{
if(lhs.length()<rhs.length())
return false;
for(int i=0;i<rhs.length(); ++i)
if(lhs[i]!=rhs[i])
return false;
return true;
}
};