中等题-二叉搜索树的性质

利用二叉搜索树中序遍历一定是升序的

/**
 * 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 {
    public boolean isValidBST(TreeNode root) {
        
    }
}

//前中后序遍历的模版

public static void preOrder(TreeNode root,List<TreeNode> list){
            //前序遍历
            if(root==null)return;

            list.add(root);
            preOrder(root.left,list);
            preOrder(root.right,list);
        }

        public static void inOrder(TreeNode root,List<TreeNode> list){
            //中序遍历
            if(root==null)return;


            inOrder(root.left,list);
            list.add(root);
            inOrder(root.right,list);
        }

        public static void postOrder(TreeNode root,List<TreeNode> list){
            //后序遍历
            if(root==null)return;


            postOrder(root.left,list);
            postOrder(root.right,list);
            list.add(root);
        }

/**
 * 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 {
    public boolean isValidBST(TreeNode root) {
        List<TreeNode> list=new ArrayList<>();
            inOrder(root,list);
            long pre=Long.MIN_VALUE;

            for(TreeNode val:list){
                if(val.val<=pre)return false;
                pre=val.val;
            }
            return true;
    }

    public  void inOrder(TreeNode root,List<TreeNode> list){
            //中序遍历
            if(root==null)return;


            inOrder(root.left,list);
            list.add(root);
            inOrder(root.right,list);
        }
}