721. Accounts Merge

https://leetcode.com/problems/accounts-merge/discuss/109158/Java-Solution-(Build-graph-+-DFS-search)

 

class Solution {
    public List<List<String>> accountsMerge(List<List<String>> accounts) {
        List<List<String>> res = new ArrayList<>();
        if(accounts == null) return res;
        HashMap<String, String> map = new HashMap<>();
        HashMap<String, Set<String>> graph = new HashMap<>();
        for(int j = 0; j < accounts.size(); j++){
            List<String> account = accounts.get(j);
            String name = account.get(0);
            for(int i = 1; i < account.size(); i++){
                if(!graph.containsKey(account.get(i))){
                    graph.put(account.get(i), new HashSet<>());
                }
                map.put(account.get(i), name);
                
                if(i != 1){
                    graph.get(account.get(i)).add(account.get(i-1));
                    graph.get(account.get(i-1)).add(account.get(i));
                }
            }
        }
        
        Set<String> visited = new HashSet<>();
        for(String email : map.keySet()){
            List<String> list = new ArrayList<>();
            String name = map.get(email);
            if(!visited.contains(email)){  //判断是否visited之后， 没有的话要加进去
                visited.add(email);
                dfs(graph, visited, list, email);
                Collections.sort(list);
                list.add(0, name);
                res.add(list);
            }   
        }
        return res;    
    }
    
    public void dfs(HashMap<String, Set<String>> graph, Set<String> visited, List<String> list, String email){
        list.add(email);
        for(String neighbor : graph.get(email)){
            if(!visited.contains(neighbor)){
                visited.add(neighbor);
                dfs(graph, visited, list, neighbor);
            }
        }
    }
}