530.二叉搜索树的最小绝对差

class Solution:
    def getMinimumDifference(self, root: Optional[TreeNode]) -> int:
        stack = []
        cur = root
        pre = None
        res = float('inf') #正无穷
        while cur or stack:
            if cur:
                stack.append(cur) #遍历节点到最底层
                cur = cur.left
            else:
                cur = stack.pop() #逐一处理节点
                if pre:
                    res = min(abs(cur.val-pre.val),res)
                pre = cur 
                cur = cur.right
        return res 

501. 二叉搜索树中的众数

class Solution:
    def findMode(self, root: Optional[TreeNode]) -> List[int]:
        stack = []
        cur = root 
        pre = None
        res = []
        maxcount,count = 0,0
        while cur or stack:
            if cur:
                stack.append(cur)
                cur = cur.left
            else:
                cur = stack.pop()
                if pre == None:
                    count =1
                elif cur.val == pre.val:
                    count +=1
                else:
                    count =1
                if count == maxcount:
                    res.append(cur.val)
                if count > maxcount:
                    maxcount=count
                    res.clear()
                    res.append(cur.val)
                pre = cur 
                cur = cur.right
        return res 

236. 二叉树的最近公共祖先

class Solution:
    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
        if not root or root == p or root == q:
            return root
        left = self.lowestCommonAncestor(root.left,p,q)
        right = self.lowestCommonAncestor(root.right,p,q)

        if left and right:
            return root
        if left:
            return left
        return right