题目地址：https://leetcode-cn.com/problems/the-earliest-moment-when-everyone-become-friends/

题目描述

Ina social group, there are N people, with unique integer ids from 0 to N-1.

Wehave a list of logs, where each logs[i] = [timestamp, id_A, id_B] contains a non-negative integer timestamp, and the ids of two different people.

Each log represents the time in which two different people became friends. Friendship is symmetric: if A is friends with B, then B is friends with A.

Let's say that person A is acquainted with person B if A is friends with B, or A is a friend of someone acquainted with B.

Return the earliest time for which every person became acquainted with every other person. Return -1 if there is no such earliest time.

Example 1:

Input: logs = [[20190101,0,1],[20190104,3,4],[20190107,2,3],[20190211,1,5],[20190224,2,4],[20190301,0,3],[20190312,1,2],[20190322,4,5]], N = 6
Output: 20190301
Explanation: 
The first event occurs at timestamp = 20190101 and after 0 and 1 become friends we have the following friendship groups [0,1], [2], [3], [4], [5].
The second event occurs at timestamp = 20190104 and after 3 and 4 become friends we have the following friendship groups [0,1], [2], [3,4], [5].
The third event occurs at timestamp = 20190107 and after 2 and 3 become friends we have the following friendship groups [0,1], [2,3,4], [5].
The fourth event occurs at timestamp = 20190211 and after 1 and 5 become friends we have the following friendship groups [0,1,5], [2,3,4].
The fifth event occurs at timestamp = 20190224 and as 2 and 4 are already friend anything happens.
The sixth event occurs at timestamp = 20190301 and after 0 and 3 become friends we have that all become friends.

Note:

1、 2<=N<=100；
2、 1<=logs.length<=10^4；
3、 0<=logs[i][0]<=10^9；
4、 0<=logs[i][1],logs[i][2]<=N-1；
5、 It'sguaranteedthatalltimestampsinlogs[i][0]aredifferent.；
6、 logsarenotnecessarilyorderedbysomecriteria.；
7、 logs[i][1]!=logs[i][2]；

题目大意

在一个社交圈子当中，有 N 个人。每个人都有一个从 0 到 N-1 唯一的 id 编号。 我们有一份日志列表 logs，其中每条记录都包含一个非负整数的时间戳，以及分属两个人的不同 id，logs[i] = [timestamp, id_A, id_B]。 每条日志标识出两个人成为好友的时间，友谊是相互的：如果 A 和 B 是好友，那么 B 和 A 也是好友。 如果 A 是 B 的好友，或者 A 是 B 的好友的好友，那么就可以认为 A 也与 B 熟识。 返回圈子里所有人之间都熟识的最早时间。如果找不到最早时间，就返回 -1 。

解题方法

并查集

提示的不能更明显了，标准的并查集。

1、 对logs按照时间排序；
2、 遍历logs，合并两个人所属的环，如果环减少到1那就是最短的时间；

C++代码如下：

class Solution {
public:
    int earliestAcq(vector<vector<int>>& logs, int N) {
        map_ = vector<int>(N);
        circle = N;
        for (int i = 0; i < N; ++i)
            map_[i] = i;
        sort(logs.begin(), logs.end(), [](vector<int>& a, vector<int>& b) {return a[0] < b[0];});
        for (auto& log : logs) {
            uni(log[1], log[2]);
            if (circle == 1)
                return log[0];
        }
        return -1;
    }
    int find(int a) {
        if (map_[a] == a)
            return a;
        return find(map_[a]);
    }
    void uni(int a, int b) {
        int pa = find(a);
        int pb = find(b);
        if (pa == pb)
            return;
        map_[pa] = pb;
        circle --;
    }
private:
    vector<int> map_;
    int circle = 0;
};

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