[LeetCode] 1101. The Earliest Moment When Everyone Become Friends
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There are n people in a social group labeled from 0
to n - 1
. You are given an array logs
where logs[i] = [timestampi, xi, yi]
indicates that xi
and yi
will be friends at the time timestampi
.
Friendship is symmetric. That means if a
is friends with b
, then b
is friends with a
. Also, person a
is acquainted with a 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. If there is no such earliest time, return -1
.
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 friends, nothing happens. The sixth event occurs at timestamp = 20190301, and after 0 and 3 become friends, we all become friends.
Example 2:
Input: logs = [[0,2,0],[1,0,1],[3,0,3],[4,1,2],[7,3,1]], n = 4 Output: 3 Explanation: At timestamp = 3, all the persons (i.e., 0, 1, 2, and 3) become friends.
Constraints:
2 <= n <= 100
1 <= logs.length <= 104
logs[i].length == 3
0 <= timestampi <= 109
0 <= xi, yi <= n - 1
xi != yi
- All the values
timestampi
are unique. - All the pairs
(xi, yi)
occur at most one time in the input.
彼此熟识的最早时间。
题目的中文翻译参见这里。题目给的 logs 是一些人两两认识的关系和他们成为朋友的时间戳,要我们求的是最早什么时候所有人都互相认识。
思路是并查集。这道题类似547题,一开始每个人都自成一组,每个人都只认识自己。之后我们遍历 logs(注意 logs 需要按照时间戳排序),根据 logs 里给出的信息我们把人放到一些连通分量里。每处理完一条 log,我们就检查连通分量的个数。当连通分量 == 1 的时候,就说明所有人都被合并在一组里了,返回此时的时间戳。
时间O(nlogn) - n个点,每个点找自己的父节点的复杂度 ≈ logn
空间O(n)
Java实现
class Solution public int earliestAcq(int[][] logs, int n) Arrays.sort(logs, (a, b) -> (a[0] - b[0])); UF uf = new UF(n); for (int[] log : logs) int time = log[0]; int a = log[1]; int b = log[2]; uf.union(a, b); if (uf.count == 1) return time; return -1; class UF int[] parent; int count; public UF(int n) parent = new int[n]; for (int i = 0; i < n; i++) parent[i] = i; count = n; public int find(int i) if (i == parent[i]) return i; return parent[i] = find(parent[i]); public void union(int p, int q) int pRoot = find(p); int qRoot = find(q); if (pRoot != qRoot) parent[pRoot] = qRoot; count--;
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