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Java-based LeetCode algorithm problem solutions, regularly updated

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2421\. Number of Good Paths

Hard

There is a tree (i.e. a connected, undirected graph with no cycles) consisting of `n` nodes numbered from `0` to `n - 1` and exactly `n - 1` edges.

You are given a **0-indexed** integer array `vals` of length `n` where `vals[i]` denotes the value of the i^th node. You are also given a 2D integer array `edges` where edges[i] = [a_i, b_i] denotes that there exists an **undirected** edge connecting nodes a_i and b_i.

A **good path** is a simple path that satisfies the following conditions:

1.  The starting node and the ending node have the **same** value.
2.  All nodes between the starting node and the ending node have values **less than or equal to** the starting node (i.e. the starting node's value should be the maximum value along the path).

Return _the number of distinct good paths_.

Note that a path and its reverse are counted as the **same** path. For example, `0 -> 1` is considered to be the same as `1 -> 0`. A single node is also considered as a valid path.

**Example 1:**

![](https://assets.leetcode.com/uploads/2022/08/04/f9caaac15b383af9115c5586779dec5.png)

**Input:** vals = [1,3,2,1,3], edges = [[0,1],[0,2],[2,3],[2,4]]

**Output:** 6

**Explanation:** There are 5 good paths consisting of a single node.

There is 1 additional good path: 1 -> 0 -> 2 -> 4.

(The reverse path 4 -> 2 -> 0 -> 1 is treated as the same as 1 -> 0 -> 2 -> 4.)

Note that 0 -> 2 -> 3 is not a good path because vals[2] > vals[0]. 

**Example 2:**

![](https://assets.leetcode.com/uploads/2022/08/04/149d3065ec165a71a1b9aec890776ff.png)

**Input:** vals = [1,1,2,2,3], edges = [[0,1],[1,2],[2,3],[2,4]]

**Output:** 7

**Explanation:** There are 5 good paths consisting of a single node.

There are 2 additional good paths: 0 -> 1 and 2 -> 3. 

**Example 3:**

![](https://assets.leetcode.com/uploads/2022/08/04/31705e22af3d9c0a557459bc7d1b62d.png)

**Input:** vals = [1], edges = []

**Output:** 1

**Explanation:** The tree consists of only one node, so there is one good path. 

**Constraints:**

*   `n == vals.length`
*   1 <= n <= 3 * 10⁴
*   0 <= vals[i] <= 10⁵
*   `edges.length == n - 1`
*   `edges[i].length == 2`
*   0 <= a_i, b_i < n
*   a_i != b_i
*   `edges` represents a valid tree.