255. Verify Preorder Sequence in Binary Search Tree - Medium

Problem:

```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.

Could you do it using only constant space complexity?
```

Solutions:

``````public class Solution {
public boolean verifyPreorder(int[] preorder) {
int low = Integer.MIN_VALUE;
Stack<Integer> stack = new Stack<Integer>();
for (int i = 0; i < preorder.length; i ++) {
if (preorder[i] < low) {
return false;
}
while(stack.size() > 0 && preorder[i] > stack.peek()) {
low = stack.pop();
}
stack.push(preorder[i]);
}
return true;
}
}
``````
``````public class Solution {
public boolean verifyPreorder(int[] preorder) {
int low = Integer.MIN_VALUE;
int index = -1;
for (int i = 0; i < preorder.length; i ++) {
if (preorder[i] < low) {
return false;
}
while (index >= 0 && preorder[i] > preorder[index]) {
low = preorder[index--];
}
preorder[++index] = preorder[i];
}
return true;
}
}
``````