Tags: "leetcode", "binary-tree", "dfs", access_time 2-min read

Check if a String Is a Valid Sequence From Root to Leaves Path in a Binary Tree

Created: April 30, 2020 by [lek-tin]

Last updated: April 30, 2020

Given a binary tree where each path going from the root to any leaf form a valid sequence, check if a given string is a valid sequence in such binary tree.

We get the given string from the concatenation of an array of integers arr and the concatenation of all values of the nodes along a path results in a sequence in the given binary tree.

Example 1:

Input: root = [0,1,0,0,1,0,null,null,1,0,0], arr = [0,1,0,1]
Output: true
Explanation: 
The path 0 -> 1 -> 0 -> 1 is a valid sequence (green color in the figure). 
Other valid sequences are: 
0 -> 1 -> 1 -> 0 
0 -> 0 -> 0

Example 2:

Input: root = [0,1,0,0,1,0,null,null,1,0,0], arr = [0,0,1]
Output: false 
Explanation: The path 0 -> 0 -> 1 does not exist, therefore it is not even a sequence.

Example 3:

Input: root = [0,1,0,0,1,0,null,null,1,0,0], arr = [0,1,1]
Output: false
Explanation: The path 0 -> 1 -> 1 is a sequence, but it is not a valid sequence.

Constraints:

1. 1 <= arr.length <= 5000
2. 0 <= arr[i] <= 9
3. Each node’s value is between [0 - 9].

Solution

Java

/**
 * 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 {
    private boolean isValid = false;
    public boolean isValidSequence(TreeNode root, int[] arr) {
        helper(root, arr, 0);
        return isValid;
    }

    private void helper(TreeNode root, int[] arr, int curr) {
        if (isValid) return;
        if (root == null) return;
        if (root != null && curr == arr.length) return;

        if (root.val != arr[curr]) return;

        if (root.left == null && root.right == null && curr == arr.length-1) {
            isValid = true;
        } else {
            helper(root.left, arr, curr+1);
            helper(root.right, arr, curr+1);
        }
    }
}