[深度优先搜索] leetcode 756 Pyramid Transition Matrix

class Solution {
public:
    vector<vector<unordered_set<char>>> pair;
    
    bool dfs(string cur, string next, int i)
    {
        if(cur.empty()) return true; // 搜到顶层了，返回true
        for(int j = i;j < cur.size();j++)
        {
            unordered_set<char>& candidate = pair[cur[j - 1] - ‘A‘][cur[j] - ‘A‘];
            if(candidate.size() == 0) return false;
            if(candidate.size() == 1) // 只有一种选择，就直接选了
            {
                next.push_back(*candidate.begin());
            }
            else
            {
                for(auto& ch : candidate) // 有多种选择，就每个都试一下
                {
                    if(dfs(cur, next + ch, j + 1))
                    {
                        return true;
                    }
                }
                return false;
            }
        }
        return dfs(next, "", 1); // 搜索下一层
    }
    bool pyramidTransition(string bottom, vector<string>& allowed) {
        pair.resize(8, vector<unordered_set<char>>(8));
        for(int i = 0;i < allowed.size(); i++)
        {
           pair[allowed[i][0] - ‘A‘][allowed[i][1] - ‘A‘].insert(allowed[i][2]);
        }
        
        return dfs(bottom, "", 1);
    }
};