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# 255     Verify Preorder Sequence in Binary Search Tree

## 255. [Verify Preorder Sequence in Binary Search Tree](https://leetcode.com/problems/verify-preorder-sequence-in-binary-search-tree/description/)

## 1. Question

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.

Consider the following binary search tree:

```
     5
    / \
   2   6
  / \
 1   3
```

**Example 1:**

```
Input: [5,2,6,1,3]

Output: false
```

**Example 2:**

```
Input: [5,2,1,3,6]

Output: true
```

**Follow up:**\
Could you do it using only constant space complexity?

## 2. Implementation

**(1) Method 1**

思路: preorder 是一个先递减再递增的顺序，维护一个单调递减栈，当前num如果大于栈顶数时，则更新min，如果当前num小于min则该序列并不是preorder序列

```java
class Solution {
    public boolean verifyPreorder(int[] preorder) {
        Stack<Integer> stack = new Stack();

        int min = Integer.MIN_VALUE;

        for (int num : preorder) {
            if (num < min) {
                return false;
            }

            while (!stack.isEmpty() && stack.peek() < num) {
                min = stack.pop();
            }
            stack.push(num);
        }
        return true;
    }
}
```

## 3. Time & Space Complexity

Method 1: 时间复杂度 O(n), 空间复杂度O(n)