Verify Preorder Sequence in Binary Search Tree in C++

Suppose we have a sequence of numbers; we have to check whether it is the correct preorder traversal sequence of a binary search tree. We can assume that each number in the sequence is unique. Consider the following binary search tree −

So, if the input is like [5,2,1,3,6], then the output will be true

To solve this, we will follow these steps −

itr := -1
low := -infinity
for initialize i := 0, when i < size of preorder, update (increase i by 1), do −
- x := preorder[i]
- if x < low, then −
  - return false
- while (itr >= 0 and preorder[itr] < x), do −
  - low := preorder[itr]
  - (decrease itr by 1)
- (increase itr by 1)
- preorder[itr] := x
return true

Example

Let us see the following implementation to get a better understanding −

Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
   bool verifyPreorder(vector<int<& preorder) {
      int itr = -1;
      int low = INT_MIN;
      for (int i = 0; i < preorder.size(); i++) {
         int x = preorder[i];
         if (x < low)
            return false;
         while (itr >= 0 && preorder[itr] < x) {
            low = preorder[itr];
            itr--;
         }
         itr++;
         preorder[itr] = x;
      }
      return true;
   }
};
main(){
   Solution ob;
   vector<int< v = {5,2,1,3,6};
   cout << (ob.verifyPreorder(v));
}