代码随想录——验证二叉搜索树（Leetcode98）

题目链接

递归

一个有效的二叉搜索树中序遍历应该是一个递增序列

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
     ArrayList<Integer> array = new ArrayList<>();
    public boolean isValidBST(TreeNode root) {
        inOrder(root);
        for(int i = 1; i < array.size(); i++){
            if(array.get(i) <= array.get(i - 1)){
                return false;
            }
        }
        return true;
    }
    // 中序遍历
    public void inOrder(TreeNode root){
        if(root == null){
            return ;
        }
        inOrder(root.left);
        array.add(root.val);
        inOrder(root.right);
    }
}