687

687. Longest Univalue Path (Medium)

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.

Companies:
Bloomberg, Google, Amazon

Related Topics:
Tree, Depth-First Search, Binary Tree

Similar Questions:

• Binary Tree Maximum Path Sum (Hard)
• Count Univalue Subtrees (Medium)
• Path Sum III (Medium)

Solution 1. DFS

// OJ: https://leetcode.com/problems/longest-univalue-path/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
    int ans = 0;
    int dfs(TreeNode *root) {
        if (!root) return 0;
        int left = dfs(root->left), right = dfs(root->right);
        if (!root->left || root->left->val != root->val) left = 0;
        if (!root->right || root->right->val != root->val) right = 0;
        ans = max(ans, 1 + left + right);
        return 1 + max(left, right);
    }
public:
    int longestUnivaluePath(TreeNode* root) {
        dfs(root);
        return max(0, ans - 1);
    }
};