方法一:前序遍历在根节点处,判断当前值是否处于(left,right)区间内,处于区间内,用root.val分别更新为左子树的右节点和右子树的左节点不处于区间内则返回False # class TreeNode: # def __init__(self, x): # self.val = x # self.left = None # self.right = None class Solution: def isValidBST(self , root: TreeNode) -> bool: return s...