2581-count-number-of-possible-root-nodes.adoc

2581. Count Number of Possible Root Nodes

{leetcode}/problems/count-number-of-possible-root-nodes/[LeetCode - 2581. Count Number of Possible Root Nodes ^]

Alice has an undirected tree with n nodes labeled from 0 to n - 1. The tree is represented as a 2D integer array edges of length n - 1 where edges[i] = [a<sub>i</sub>, b<sub>i</sub>] indicates that there is an edge between nodes a<sub>i</sub> and b<sub>i</sub> in the tree.

Alice wants Bob to find the root of the tree. She allows Bob to make several guesses about her tree. In one guess, he does the following:

• Chooses two distinct integers u and v such that there exists an edge [u, v] in the tree.

• He tells Alice that u is the parent of v in the tree.

Bob’s guesses are represented by a 2D integer array guesses where guesses[j] = [u<sub>j</sub>, v<sub>j</sub>] indicates Bob guessed u<sub>j</sub> to be the parent of v<sub>j</sub>.

Alice being lazy, does not reply to each of Bob’s guesses, but just says that at least k of his guesses are true.

Given the 2D integer arrays edges, guesses and the integer k, return the number of possible nodes that can be the root of Alice’s tree. If there is no such tree, return 0.

Example 1:

<img alt="" src="https://assets.leetcode.com/uploads/2022/12/19/ex-1.png" style="width: 727px; height: 250px;" />

Input: edges = [[0,1],[1,2],[1,3],[4,2]], guesses = [[1,3],[0,1],[1,0],[2,4]], k = 3
Output: 3
Explanation:
Root = 0, correct guesses = [1,3], [0,1], [2,4]
Root = 1, correct guesses = [1,3], [1,0], [2,4]
Root = 2, correct guesses = [1,3], [1,0], [2,4]
Root = 3, correct guesses = [1,0], [2,4]
Root = 4, correct guesses = [1,3], [1,0]
Considering 0, 1, or 2 as root node leads to 3 correct guesses.

Example 2:

<img alt="" src="https://assets.leetcode.com/uploads/2022/12/19/ex-2.png" style="width: 600px; height: 303px;" />

Input: edges = [[0,1],[1,2],[2,3],[3,4]], guesses = [[1,0],[3,4],[2,1],[3,2]], k = 1
Output: 5
Explanation:
Root = 0, correct guesses = [3,4]
Root = 1, correct guesses = [1,0], [3,4]
Root = 2, correct guesses = [1,0], [2,1], [3,4]
Root = 3, correct guesses = [1,0], [2,1], [3,2], [3,4]
Root = 4, correct guesses = [1,0], [2,1], [3,2]
Considering any node as root will give at least 1 correct guess.

Constraints:

• edges.length == n - 1

• 2 ⇐ n ⇐ 105

• 1 ⇐ guesses.length ⇐ 105

• 0 ⇐ a<sub>i</sub>, b<sub>i</sub>, u<sub>j</sub>, v<sub>j</sub> ⇐ n - 1

• a<sub>i</sub> != b<sub>i</sub>

• u<sub>j</sub> != v<sub>j</sub>

• edges represents a valid tree.

• guesses[j] is an edge of the tree.

• guesses is unique.

• 0 ⇐ k ⇐ guesses.length

思路分析

一刷

link:{sourcedir}/_2581_CountNumberOfPossibleRootNodes.java[role=include]

参考资料