# 687 Longest Univalue Path

## 687. Longest Univalue Path

## 1. Question

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

**Note:**The length of path between two nodes is represented by the number of edges between them.

**Example 1:**

Input:

```
              5
             / \
            4   5
           / \   \
          1   1   5
```

Output:

```
2
```

**Example 2:**

Input:

```
              1
             / \
            4   5
           / \   \
          4   4   5
```

Output:

```
2
```

**Note:**The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

## 2. Implementation

**(1) DFS**

思路: 先用递归地方法分别得到Left children和Right children的longest univalue path, 然后判断当前的node value是否和left children或者right children的相同，如果是的话，之前得到的longest univalue path加1， 然后与当前得到的最长的univalue path比较

```java
class Solution {
    public int longestUnivaluePath(TreeNode root) {
        int[] res = new int[1];
        getLongestUnivaluePath(root, res);
        return res[0];
    }

    public int getLongestUnivaluePath(TreeNode node, int[] res) {
        if (node == null) {
            return 0;
        }

        int leftPath = getLongestUnivaluePath(node.left, res);
        int rightPath = getLongestUnivaluePath(node.right, res);
        int leftPathFromNode = 0, rightPathFromNode = 0;

        if (node.left != null && node.left.val == node.val) {
            leftPathFromNode = leftPath + 1;
        }

        if (node.right != null && node.right.val == node.val) {
            rightPathFromNode = rightPath + 1;
        }

        res[0] = Math.max(res[0], leftPathFromNode + rightPathFromNode);
        return Math.max(leftPathFromNode, rightPathFromNode);
    }
}
```

## 3. Time & Space Complexity

DFS: 时间复杂度: O(n), 空间复杂度: O(h)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://protegejj.gitbook.io/my-algorithm-summary/data-structure/tree/687-longest-univalue-path.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.