0687. Longest Univalue Path

https://leetcode.com/problems/longest-univalue-path

Description

Given the root of a binary tree, return 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.

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

Example 1:

**Input:** root = [5,4,5,1,1,5]
**Output:** 2

Example 2:

**Input:** root = [1,4,5,4,4,5]
**Output:** 2

Constraints:

• The number of nodes in the tree is in the range [0, 104].

• -1000 <= Node.val <= 1000

• The depth of the tree will not exceed 1000.

ac

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int max;
    public int longestUnivaluePath(TreeNode root) {
        max = 0;
        unilateralLen(root);
        return max;
    }

    private int unilateralLen(TreeNode root) {
        // exit
        if (root == null) return 0;

        int left = unilateralLen(root.left);
        int right = unilateralLen(root.right);

        int l = root.left != null && root.val == root.left.val ? left + 1 : 0;
        int r = root.right != null && root.val == root.right.val ? right + 1 : 0;

        max = Math.max(max, l + r); // update max

        return Math.max(l, r);
    }
}