leecode 树是否是平衡树 java

https://oj.leetcode.com/problems/validate-binary-search-tree/

1.中序遍历是否有序

/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    private int a=-(1<<31);

  

    public boolean isValidBST(TreeNode root) {

        

        

      return middle(root);

       

        

    }

    public boolean middle(TreeNode root)

    {

        if(root==null) return true;

        if(!middle(root.left)) return false;

        if(root.val<=a) 

        {

            return false;

        

        }

        

        a=root.val;

        return middle(root.right);

        

        

    }

}

　　2.记住每个节点的范围 开始时候（min,max）设为int最大，最小，然后在左子树上范围为（min, root.val） 判断节点是否在范围；

 

 

/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    public boolean isValidBST(TreeNode root) {

        

        

        int min=-(1<<31);

        int max=1<<31-1;

       return isValid(root,min,max);

        

        

    }

    public boolean isValid(TreeNode root,int min,int max)

    {

        if(root==null) return true;

        if(root.val>min&&root.val<max)

        {

            return isValid(root.left,min,root.val)&&isValid(root.right,root.val,max);

            

        }

        return false;

        

        

    }

}