Leetcode "word break"

My first solution was DFS-tle. Apparently, DFS with TLE usually means DP.

1D DP + rolling array:

class Solution {public:    bool wordBreak(string s, unordered_set<string> &dict) {        int len = s.length();        int lend= dict.size();        if(len == 0 || lend == 0) return false;                //    Init        vector<bool> rec0, rec1;        rec0.resize(len); rec1.resize(len);        std::fill(rec0.begin(), rec0.end(), false);        std::fill(rec1.begin(), rec1.end(), false);        //        vector<bool> &pre = rec0;        vector<bool> &post = rec1;        //    Start from 0        for(int i = 1; i <= len; i ++)        {            string sub = s.substr(0, i);            if(dict.find(sub) != dict.end())            {                pre[i-1] = true;                if(i == len) return true;            }        }        for(int i = 1; i < len; i ++)    // vertical        {            for(int j = 0; j < len; j ++)    // horizontal            {                if(pre[j])                {                    for(int k = 1; k < len - j; k ++)                    {                        string sub = s.substr(j + 1, k);                        if(dict.find(sub) != dict.end())                        {                            post[j + k] = true;                            if((j + k + 1) == len) return true;                        }                    }                }            }            //    Rolling array            if(i % 2)            {                pre = rec1; post = rec0;            }            else            {                pre = rec0; post = rec1;            }        }        return false;    }};