leetcode solution 98 Valid Binary Search Tree
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 2018-08-15 19:23:00  

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:

1
2
3
4
5
Input:
    2
   / \
  1   3
Output: true

Example 2:

1
2
3
4
5
6
7
8
    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.
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