【leetcode 98】Validate Binary Search Tree 二叉搜索树

题目

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

    2
   / \
  1   3

Input: [2,1,3]
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6

Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

解题思路

由于二叉搜索树本身的性质得知，对二叉搜索树进行中序遍历后，得到的数组就是一个从小到大排好序的数组，且每个节点的值不相同。

所以根据以上特性，对该树进行中序遍历，将遍历后的结果进行判断，如果结果是一个从小到大排好序的数组，那么该树就是一个二叉搜索树

代码实现

    public boolean isValidBST(TreeNode root) {
        List<Integer> list = inorderTraversal(root);
        for (int i = 0; i < list.size() - 1; i++) {
            if (list.get(i) >= list.get(i + 1)) {
                return false;
            }
        }
        return true;
    }

    public List<Integer> inorderTraversal(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }
        List<Integer> list = inorderTraversal(root.left);
        list.add(root.val);
        list.addAll(inorderTraversal(root.right));
        return list;
    }