Leetcode: Word Break

Given a string s and a dictionary of words dict, determine if s can be segmented into a space-separated sequence of one or more dictionary words.

For example, given

s = "leetcode",
dict = ["leet", "code"].

Return true because "leetcode" can be segmented as "leet code".

第一种方法：递归（超时）Time Limit Exceeded

思路：从s的第一个字母向后匹配，如果i前面的前缀可以匹配，就看s字符串i以后的后缀是否匹配

Last executed input:

"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab", ["a","aa","aaa","aaaa","aaaaa","aaaaaa","aaaaaaa","aaaaaaaa","aaaaaaaaa","aaaaaaaaaa"]

bool wordBreak(string s, unordered_set<string> &dict) {
        // Note: The Solution object is instantiated only once.
        if(s.length() < 1) return true;
		bool flag = false;
		for(int i = 1; i <= s.length(); i++)
		{
			string tmpstr = s.substr(0,i);
			unordered_set<string>::iterator it = dict.find(tmpstr);
			if(it != dict.end())
			{
				if(tmpstr.length() == s.length())return true;
				flag = wordBreak(s.substr(i),dict);
			}
			if(flag)return true;
		}
		return false;
    }

第二种方法：dpAccepted

思路：从s的第一个字母向后匹配，如果i前面的前缀可以匹配，就看s字符串i以后的后缀是否匹配，在找后缀是否匹配时添加了记忆功能。

bool wordBreakHelper(string s, unordered_set<string> &dict,set<string> &unmatch) {
        if(s.length() < 1) return true;
		bool flag = false;
		for(int i = 1; i <= s.length(); i++)
		{
			string prefixstr = s.substr(0,i);
			unordered_set<string>::iterator it = dict.find(prefixstr);
			if(it != dict.end())
			{
				string suffixstr = s.substr(i);
				set<string>::iterator its = unmatch.find(suffixstr);
				if(its != unmatch.end())continue;
				else{
					flag = wordBreakHelper(suffixstr,dict,unmatch);
					if(flag) return true;
					else unmatch.insert(suffixstr);
				}
			}
		}
		return false;
    }
	bool wordBreak(string s, unordered_set<string> &dict) {
        // Note: The Solution object is instantiated only once.
        int len = s.length();
		if(len < 1) return true;
		set<string> unmatch;
		return wordBreakHelper(s,dict,unmatch);
    }

dp改进：dict中的单词有的长有的短，当prefixstr串小于最短串时就不匹配了，当prefixstr串大于最长的串时也不用匹配了。多谢@阿桂爱清净

bool wordBreakHelper(string s,unordered_set<string> &dict,set<string> &unmatched,int mn,int mx) {
        if(s.size() < 1) return true;
		int i = mx < s.length() ? mx : s.length();
		for(; i >= mn ; i--)
		{
			string preffixstr = s.substr(0,i);
			if(dict.find(preffixstr) != dict.end()){
				string suffixstr = s.substr(i);
				if(unmatched.find(suffixstr) != unmatched.end())
					continue;
				else
					if(wordBreakHelper(suffixstr, dict, unmatched,mn,mx))
						return true;
					else
						unmatched.insert(suffixstr);
			}
		}
        return false;
    }
	bool wordBreak(string s, unordered_set<string> &dict) {
        // Note: The Solution object is instantiated only once.
		if(s.length() < 1) return true;
		if(dict.empty()) return false;
		unordered_set<string>::iterator it = dict.begin();
		int maxlen=(*it).length(), minlen=(*it).length();
		for(it++; it != dict.end(); it++)
			if((*it).length() > maxlen)
				maxlen = (*it).length();
			else if((*it).length() < minlen)
				minlen = (*it).length();
        set<string> unmatched;
		return wordBreakHelper(s,dict,unmatched,minlen,maxlen);
    }