五. 搜索二叉树 3 Validate Binary Search Tree

思路：

1. 使用中序遍历，将所有数值存在一个数组里，如果是按increasing顺序排列，则是一个二叉搜索树。

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""


class Solution:
    """
    @param: root: The root of binary tree.
    @return: True if the binary tree is BST, or false
    """
    def isValidBST(self, root):
        # write your code here
        self.list = []
        if not root:
            return True
        def build_list(root):
            if root:
                build_list(root.left)
                self.list.append(root.val)
                build_list(root.right)
            return
            
        build_list(root)
        
        for i in range(0, len(self.list)-1):
            if self.list[i]< self.list[i+1]:
                continue
            else:
                return False
            
            
        return True