# To check if array represents preorder traversal of a binary tree

Problem: For a given array of numbers, check if it represents preorder traversal of a binary search tree.

Problem Explanation:

Input: arr={51, 41, 45, 81, 111}, it represents preorder traversal of binary search tree below

```     51
/   \
41    81
\      \
45     111```

Output: True

Input: arr={20, 25, 1}, it represents the tree below which is not a binary search tree

```    20
\
25
/
1```

Output: False

Java Implementation

```public class IsBSTRepresentation {
boolean isBSTRepresentation(int[] arr) {
Stack<Integer> stack = new Stack<Integer>();
int root = Integer.MIN_VALUE;

for (int i = 0; i < arr.length; i++) {
if (arr[i] < root) {
return false;
}

while (!stack.empty() && (Integer) stack.peek() < arr[i]) {
root = (Integer) stack.peek();
stack.pop();
}
stack.push(arr[i]);
}
return true;
}

public static void main(String[] args) {

IsBSTRepresentation isBSTRepresentation = new IsBSTRepresentation();
System.out.println(isBSTRepresentation.isBSTRepresentation(new int[]{20, 25, 1}));

}
}
```

TimeComplexity: O(n)

Note: If you find any other better way to approach to this problem or you find any issue/error in above code snippets/approaches – please share it in the comments section below and get a chance to enrol free of cost for an online Live Data Structure and Algorithm course (specially designed for interview preparation for software companies like Amazon, Google, Facebook, FlipKart, SnapDeal, HealthKart…)