Check if a given array can represent Preorder Traversal of Binary Search Tree in C++

Suppose we have a list of elements in an array, we have to check, whether the elements can be the preorder traversal of a binary search tree or not. Suppose a sequence is like {40, 30, 35, 80, 100}, then the tree will be like −

We can solve this using a stack. We have to follow these steps to solve this problem.

  • Define empty stack
  • set root as negative infinity
  • for every element in preorder sequence, do the following −
    • if the element is smaller than current root, return false
    • Keep removing elements from stack while the element is greater than stack top, and make the lastly removed element as root.
    • push the element into stack


#include <iostream>
#include <stack>
using namespace std;
bool isValidPreorder(int pre[], int n) {
   stack<int> stk;
   int root = INT_MIN;
   for (int i=0; i<n; i++) {
      if (pre[i] < root)
         return false;
      while (!stk.empty() &&<pre[i]) {
         root =;
   return true;
int main() {
   int pre[] = {40, 30, 35, 80, 100};
   int n = sizeof(pre)/sizeof(pre[0]);
   if(isValidPreorder(pre, n))
      cout << "This can form BST";
      cout << "This can not form BST";


This can form BST