LCR 114. 火星词典

//https://leetcode.cn/problems/Jf1JuT/
const int N=26,M=110;
class Solution {
private:
    int head[N],Next[M],to[M],cnt;
    int indegree[N];
    int h,t;
    int q[N];
    string ans;
    void addEdge(int u,int v){
        Next[cnt]=head[u];
        to[cnt]=v;
        head[u]=cnt++;
    }
    void build(){
        h=t=0;
        cnt=1;
        memset(indegree,-1,sizeof indegree);
        memset(head,0,sizeof head);
    }
    bool topoSort(){
        int kinds=0;
        for(int i=0;i<26;i++){
            if(indegree[i]!=-1) kinds++;
            if(indegree[i]==0) q[t++]=i;
        }
        int fill=0;
        while(h<t){
            int u=q[h++];
            ans.push_back(u+'a'),fill++;
            for(int ei=head[u],v;ei;ei=Next[ei]){
                v=to[ei];
                if(--indegree[v]==0) q[t++]=v;
            }
        }
        return fill==kinds;
    }
public:
    string alienOrder(vector<string>& words) {
        build();
        for(int i=0;i<words.size();i++){
            for(int j=0;j<words[i].size();j++){
                indegree[words[i][j]-'a']=0;
            }
        }
        for(int i=0;i<words.size()-1;i++){
            string cur=words[i],nxt=words[i+1];
            int len=min(cur.size(),nxt.size());
            int j=0;
            for(;j<len;j++){
                if(cur[j]!=nxt[j]){
                    addEdge(cur[j]-'a',nxt[j]-'a');
                    indegree[nxt[j]-'a']++;
                    break;
                }
            }
            if(j<cur.size()&&j==nxt.size()) return "";
        }
        if(!topoSort()) ans.clear();
        return ans;
    }
};