298. Binary Tree Longest Consecutive Sequence

Published on July 17, 2023

Question

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.

Example 1:

Input: root = [1,null,3,2,4,null,null,null,5]
Output: 3
Explanation: Longest consecutive sequence path is 3-4-5, so return 3.

Example 2:

Input: root = [2,null,3,2,null,1]
Output: 2
Explanation: Longest consecutive sequence path is 2-3, not 3-2-1, so return 2.

Algorithm

Another typical binary tree question. Using preorder traversal, we calculate the max length each time we reach a new node until we reach the leaf nodes.

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int res = 0;
    public int longestConsecutive(TreeNode root) {
        helper(root, 1);
        return res;
    }
    
    private void helper(TreeNode root, int cur) {
        if (root == null) return;
        res = Math.max(cur, res);
        if (root.left == null && root.right == null) {
            return;
        }
        if (root.left != null) {
            if (root.left.val == root.val + 1) {
                helper(root.left, cur + 1);
            } else {
                helper(root.left, 1);
            }
        }
        if (root.right != null) {
            if (root.right.val == root.val + 1) {
                helper(root.right, cur + 1);
            } else {
                helper(root.right, 1);
            }
        }
    }
}