255 Verify Preorder Sequence in Binary Search Tree

255. Verify Preorder Sequence in Binary Search Tree

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

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)