298

/**
 * 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 {
int ans = 0;
public:
    int longestConsecutive(TreeNode* root) {
        dfs(root, 1, -1);
        return ans;
    }

    void dfs(TreeNode* root, int len, int parentVal){
        if(!root) return;
        len = (root->val == parentVal+1) ? len : 1;
        ans =  max(len, ans);
        dfs(root->left, len+1, root->val);
        dfs(root->right, len+1, root->val);
    }
};