leetcode-easy-trees-98. Validate Binary Search Tree-NO

mycode   不会

注意：root的值要比左子树上所有的数大

 参考

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

# in valid BST, in-order taversal would go
# from node with the smallest value  (left most leaf)
# up to the node with the biggest value (right most leaf)

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
          return True

        self.inorder = []
        
        def _dfs(node):
            
            if node.left:
              _dfs(node.left)
            self.inorder.append(node)
            if node.right:
              _dfs(node.right)
            
      
        _dfs(root)
        
        prev = self.inorder[0].val
        for node in self.inorder[1:]:
            print(node.val)
            if node.val > prev:
                prev = node.val
            else:
                return False
        return True

 

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        
        def inorder(root, output):
            if not root: return
            if root.left: inorder(root.left, output)
            output += [root.val]
            if root.right: inorder(root.right, output)
        output=[]
        inorder(root, output)
        for i in range(1,len(output)):
            if output[i]<=output[i-1]:
                return False
        return True

 

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        import sys
        
        min_val = -sys.maxsize
        max_val = sys.maxsize
        
        return self.isValidBST_Util(root, min_val, max_val)
    
    def isValidBST_Util(self, root, min_val, max_val):
        if not root: return True
        
        if root.val > min_val and \
            root.val < max_val and \
            self.isValidBST_Util(root.left, min_val, root.val) and \
            self.isValidBST_Util(root.right, root.val, max_val):
            return True
        else:
            return False