[leetcode]Validate Binary Search Tree

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：

public class Solution {
   public boolean isValidBST(TreeNode root) {
        if(root == null || (root.left == null && root.right == null)) return true;
        return isValid(root, Integer.MAX_VALUE, Integer.MIN_VALUE);
    }
    private boolean isValid(TreeNode root,int biggest,int smallest){
        if(root == null ) return true;
        if(root.val >= biggest || root.val <= smallest) return false;
        else 
            return isValid(root.left, root.val, smallest) && isValid(root.right, biggest, root.val);
    }
}