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🐳(dfs): 98. 验证二叉搜索树

补充 while 中序遍历

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`@@ -231,6 +231,35 @@ func helper(root *TreeNode, low int, high int) bool {`
`231`	`231`	`}`
`232`	`232`	```
`233`	`233`
	`234`	`+### 解三:栈辅助中序遍历`
	`235`	`+`
	`236`	+```python
	`237`	`+# Definition for a binary tree node.`
	`238`	`+# class TreeNode:`
	`239`	`+# def __init__(self, val=0, left=None, right=None):`
	`240`	`+# self.val = val`
	`241`	`+# self.left = left`
	`242`	`+# self.right = right`
	`243`	`+class Solution:`
	`244`	`+ def isValidBST(self, root: TreeNode) -> bool:`
	`245`	`+ stack = []`
	`246`	`+ pre = float('-inf')`
	`247`	`+ while root is not None or len(stack) > 0:`
	`248`	`+ # 左侧压入栈`
	`249`	`+ while root is not None:`
	`250`	`+ stack.append(root)`
	`251`	`+ root = root.left`
	`252`	`+ # 弹出栈顶`
	`253`	`+ top = stack.pop()`
	`254`	`+ # 判断终序遍历前 1 个数是否小于当前数`
	`255`	`+ if top.val <= pre:`
	`256`	`+ return False`
	`257`	`+ pre = top.val`
	`258`	`+ root = top.right`
	`259`	`+`
	`260`	`+ return True`
	`261`	+```
	`262`	`+`
`234`	`263`	`<!-- tabs:end -->`
`235`	`264`
`236`	`265`	`## 105. 从前序与中序遍历序列构造二叉树`

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