Leetcode 255: Verify Preorder Sequence in Binary Search Tree

Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree.

You may assume each number in the sequence is unique.

Follow up:
Could you do it using only constant space complexity?

 

public class Solution {
    public bool VerifyPreorder(int[] preorder) {
        if (preorder == null || preorder.Length < 2) return true;
        
        var stack = new Stack<int>();
        var low = Int32.MinValue;
        
        for (int i = 0; i < preorder.Length; i++)
        {
            if (preorder[i] < low) return false;
            
            while (stack.Count > 0 && preorder[i] > stack.Peek())
            {
                low = stack.Pop();
            }
            
            stack.Push(preorder[i]);
        }
        
        return true;
    } 
}

// time limit exceeded
public class Solution1 {
    public bool VerifyPreorder(int[] preorder) {
        if (preorder == null || preorder.Length < 2) return true;
        
        return DFS(preorder, 0, preorder.Length - 1);
    }
    
    private bool DFS(int[] preorder, int start, int end)
    {
        if (start >= end) return true;      
        int right = start + 1;
        
        while (right <= end)
        {
            if (preorder[right] > preorder[start])
            {
                break;
            }
            
            right++;
        }
        
        for (int i = right + 1; i <= end; i++)
        {
            if (preorder[i] < preorder[start])
            {
                return false;
            }
        }
        
        return DFS(preorder, start + 1, right - 1) && DFS(preorder, right, end);
    }
}