255. Verify Preorder Sequence in Binary Search Tree

Given an array of unique integers preorder, return true if it is the correct preorder traversal sequence of a binary search tree.

1
2
3
4
5
6
7
8
9
Example 1:

Input: preorder = [5,2,1,3,6]
Output: true

Example 2:

Input: preorder = [5,2,6,1,3]
Output: false

ex1

Constraints:

1 <= preorder.length <= 10^4
1 <= preorder[i] <= 10^4
All the elements of preorder are unique.

Solution

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
class Solution {
    public boolean verifyPreorder(int[] preorder) {
        Stack<Integer> stack = new Stack<>();
        int lowerBound = Integer.MIN_VALUE;
        for (int val: preorder) {
            
            while (stack.size() > 0 && val > stack.peek()) {
                lowerBound = stack.pop();
            }
            
            if (val < lowerBound) {
                return false;
            }
            stack.push(val);
        }
        return true;
    }
}

Solution#

Solution