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[leetcode]Validate Binary Search Tree

2024-07-15 03:47:35   226人阅读

Validate Binary Search Tree

 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.

算法思路：

思路1：求出其中序遍历序列，判断其实是否为递增序列，空间复杂度为O(n)

 

思路2：二分法。每一个点的上下界都是确定的。

代码如下：

 1 public class Solution { 2    public boolean isValidBST(TreeNode root) { 3         if(root == null || (root.left == null && root.right == null)) return true; 4         return isValid(root, Integer.MAX_VALUE, Integer.MIN_VALUE); 5     } 6     private boolean isValid(TreeNode root,int biggest,int smallest){ 7         if(root == null ) return true; 8         if(root.val >= biggest || root.val <= smallest) return false; 9         else 10             return isValid(root.left, root.val, smallest) && isValid(root.right, biggest, root.val);11     }12 }

 


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