298. Binary Tree Longest Consecutive Sequence

Difficulty: Medium

Given the root of a binary tree, return the length of the longest consecutive sequence path.

A consecutive sequence path is a path where the values increase by one along the path.

Note that the path can start at any node in the tree, and you cannot go from a node to its parent in the path.

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int longestConsecutive(TreeNode* root) {
        DFS(root);
        return maxlen_;
    }

private:
    int DFS(TreeNode* node) {
        int left = 0, right = 0;
        if (node->left != nullptr) {
            if (node->left->val == node->val + 1) {
                left = DFS(node->left);
            }
            else {
                DFS(node->left);
            }
        }
        if (node->right != nullptr) {
            if (node->right->val == node->val + 1) {
                right = DFS(node->right);
            }
            else {
                DFS(node->right);
            }
        }
        int res = 1 + std::max(left, right);
        maxlen_ = std::max(maxlen_, res);
        return res;
    }
    
    int maxlen_ = 0;
};

Leetcode-298