721. Accounts Merge

721. Accounts Merge


Union find 
https://www.youtube.com/watch?v=VJnUwsE4fWA

https://www.youtube.com/watch?v=ID00PMy0-vE&t=572s



Uf code in java 
http://zxi.mytechroad.com/blog/data-structure/sp1-union-find-set/

这个把图画一下，不然很快就忘了





Input: 
accounts =
  [["John", "johnsmith@mail.com", "john00@mail.com"],
   ["John", "johnnybravo@mail.com"],
   ["John", "johnsmith@mail.com", "john_newyork@mail.com"],
   ["Mary", "mary@mail.com"]]
   
  
  
Output:
[["John", 'john00@mail.com', 'john_newyork@mail.com', 'johnsmith@mail.com'], 
 ["John", "johnnybravo@mail.com"], 
 ["Mary", "mary@mail.com"]]
 



Explanation: 
The first and third John's are the same person as they have the common email "johnsmith@mail.com".
The second John and Mary are different people as none of their email addresses are used by other accounts.
We could return these lists in any order, for example the answer [['Mary', 'mary@mail.com'], ['John', 'johnnybravo@mail.com'], 
['John', 'john00@mail.com', 'john_newyork@mail.com', 'johnsmith@mail.com']] would still be accepted.

class Solution {
    public List<List<String>> accountsMerge(List<List<String>> accounts) {
      UnionFind uf = new UnionFind();
      Map<String, String> emailToName = new HashMap<>();
      Map<String, Integer> emailToID = new HashMap<>();
      int id = 0; // later use for increment the id 
      for(List<String> account : accounts){
        for(int i = 1; i < account.size(); i++){
          if(!emailToName.containsKey(account.get(i))){
            emailToName.put(account.get(i), account.get(0));
            emailToID.put(account.get(i), id++);
            int firstEmail = emailToID.get(account.get(1)); // the first email's id
            int other = emailToID.get(account.get(i)); // other emails id from the same account
            uf.union(firstEmail, other); // in the same account, we union the first email with the rest 
          }
        }
      }
      
      Map<Integer, List<String>> ans = new HashMap<>(); // key: ultimate root,val: the list of emails belong to that root
      for(String email : emailToID.keySet()){ //get all the distinct email,then get their id, then get their ultimate root id 
        int root = uf.find(emailToID.get(email));
        if(!ans.containsKey(root)){
          ans.put(root, new ArrayList<String>()); ////
        }
        ans.get(root).add(email);
      }
      
      for(List<String> emails: ans.values()){ // grouped unsorted emails without names 
        Collections.sort(emails);
        emails.add(0, emailToName.get(emails.get(0)));
      }
      return new ArrayList(ans.values()); ///////
    }
    class UnionFind{
      int[] id;
      
      public UnionFind(){
        id = new int[10001];
        for(int i = 0; i <= 10000l; i++){
          id[i] = i;
        }
      }
      
      public int find(int x){
        while(id[x] != x){
          x = id[x];
        }
        return x;
      }
      
      public void union(int p, int q){
        int proot = find(p);
        int qroot = find(q);
        id[qroot] = proot;
        }
      }
}
=======================================================================================================

email to id hashmap 



email: "johnsmith@mail.com", "john00@mail.com", "johnnybravo@mail.com", "john_newyork@mail.com", "mary@mail.com"
 id :     0                       1                   2                         3                      4

===========================================================================================================
   
   
email to name  hashmap



email: "johnsmith@mail.com", "john00@mail.com", "johnnybravo@mail.com", "john_newyork@mail.com", "mary@mail.com"
name:      john                   john               john                     john                    mary





===========================================================================================================


int[] id:  0  0  2  0  4 
  index.   0. 1. 2  3. 4
  
  
  
===========================================================================================================
  
  
ans hashmap 

ultimate root             0                            2                       4
emails          "johnsmith@mail.com"         "johnnybravo@mail.com     "mary@mail.com"
                 "john00@mail.com"
             john_newyork@mail.com

 

Given a list accounts, each element accounts[i] is a list of strings, where the first element accounts[i][0] is a name, and the rest of the elements are emails representing emails of the account.

Now, we would like to merge these accounts. Two accounts definitely belong to the same person if there is some email that is common to both accounts. Note that even if two accounts have the same name, they may belong to different people as people could have the same name. A person can have any number of accounts initially, but all of their accounts definitely have the same name.

After merging the accounts, return the accounts in the following format: the first element of each account is the name, and the rest of the elements are emails in sorted order. The accounts themselves can be returned in any order.

Example 1:

Input: 

accounts = 
[["John", "johnsmith@mail.com", "john00@mail.com"], 
["John", "johnnybravo@mail.com"], 
["John", "johnsmith@mail.com", "john_newyork@mail.com"],
 【"Mary", "mary@mail.com"]]



Output: 
[["John", 'john00@mail.com', 'john_newyork@mail.com', 'johnsmith@mail.com'],
["John", "johnnybravo@mail.com"], 
["Mary", "mary@mail.com"]]



Explanation: 

The first and third John's are the same person as they have the common email "johnsmith@mail.com".
The second John and Mary are different people as none of their email addresses are used by other accounts.
We could return these lists in any order, for example the answer [['Mary', 'mary@mail.com'], ['John', 'johnnybravo@mail.com'], 
['John', 'john00@mail.com', 'john_newyork@mail.com', 'johnsmith@mail.com']] would still be accepted.