HàPhan 河

Check If a String Is a Valid Sequence from Root to Leaves Path in a Binary Tree

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 <= arr.length <= 5000`
• `0 <= arr[i] <= 9`
• Each node's value is between `[0 - 9]`.

Solution

Preorder Traversal with increasement of array index.
When reaching last index, check node is leaf or not.

``````class Solution {
func isValidSequence(_ root: TreeNode?, _ arr: [Int]) -> Bool {
return helper(root, arr, 0)
}

func helper(_ node: TreeNode?, _ arr: [Int], _ index: Int) -> Bool {
if index == arr.count - 1 {
return node?.left == nil && node?.right == nil && node?.val == arr[index]
}

if node?.val != arr[index] {
return false
}

return helper(node?.left, arr, index + 1) || helper(node?.right, arr, index + 1)
}
}
``````