leetcode-98 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.

 

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.

 

保证为二叉搜索树，要求左节点小于当前节点，且当前节点小于右节点。并且所有的左节点数都要小于一开始的节点数。

    public boolean isValidBST(TreeNode root) {         return isValidBST(root,null,null);     }          public boolean isValidBST(TreeNode root,Integer low,Integer high) {         if (root == null) {             return true;         }         int val = root.val;         if(low!=null && val <= low) {             return false;         }                  if(high!=null && high <= val) {             return false;         }                  if(!isValidBST(root.left,low,val)) {             return false;         }                  if(!isValidBST(root.right,val,high)) {             return false;         }         

                 return true;              }