721. 账户合并

解法一：并查集

参考官方题解😥。

import java.util.*;
import java.util.Map.Entry;
class Solution {
    public List<List<String>> accountsMerge(List<List<String>> accounts) {
        Map<String, Integer> emailToIndex = new HashMap<>();
        Map<String, String> emailToName = new HashMap<>();
        int emailNum = 0;
        for (List<String> account : accounts) {
            String name = account.get(0);
            for (int i = 1; i < account.size(); ++i) {
                String email = account.get(i);
                if (!emailToIndex.containsKey(email)) {
                    emailToIndex.put(email, emailNum++);
                    emailToName.put(email, name);
                }
            }
        }
        UnionFind uf = new UnionFind(emailNum);
        for (List<String> account : accounts) {
            String firstEmail = account.get(1);
            int firstIndex = emailToIndex.get(firstEmail);
            for (int i = 2; i < account.size(); ++i) {
                String secondEmail = account.get(i);
                int secondIndex = emailToIndex.get(secondEmail);
                uf.union(firstIndex, secondIndex);
            }
        }
        Map<Integer, List<String>> indexToEmails = new HashMap<>();
        for (Map.Entry<String, Integer> entry : emailToIndex.entrySet()) {
            String email = entry.getKey();
            int index = uf.find(entry.getValue());
            List<String> list = indexToEmails.getOrDefault(index, new ArrayList<String>());
            list.add(email);
            indexToEmails.put(index, list);
        }
        List<List<String>> ans = new ArrayList<>();
        for (Entry<Integer, List<String>> entry : indexToEmails.entrySet()) {
            String name = emailToName.get(entry.getValue().get(0));
            List<String> list = entry.getValue();
            Collections.sort(list);
            list.add(0, name);
            ans.add(list);
        }
        return ans;
    }
}
class UnionFind {
    private int[] father;
    public UnionFind(int n) {
        father = new int[n];
        for (int i = 0; i < n; ++i) {
            father[i] = i;
        }
    }
    public void union(int x, int y) {
        father[x] = find(x);
        father[y] = find(y);
        father[father[x]] = father[y];
    }
    public int find(int x) {
        return father[x] == x ? x : find(father[x]);
    }
}