代码随想录算法训练营day18 |530.二叉搜索树的最小绝对差 501.二叉搜索树中的众数 236. 二叉树的最近公共祖先

tristan241001 / 2024-10-23 / 原文

学习资料：https://programmercarl.com/0530.二叉搜索树的最小绝对差.html

530.二叉搜索树的最小绝对差（双指针法，pre&cur，设置最小差值初始为无穷大，当差值<最小差值就更新最小差值）

点击查看代码

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def __init__(self):
        self.result = float('inf')  # 设置最小值的初值为无穷大
        self.pre = None
    def getMinimumDifference(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return
        self.getMinimumDifference(root.left)
        # 因为中序遍历，这里root.val肯定大于self.pre.val
        if self.pre is not None:
            self.result = min(self.result, root.val-self.pre.val) 
        self.pre = root
        self.getMinimumDifference(root.right)
        return self.result

501.二叉搜索树中的众数（加两个函数,init & searchBST；双指针，当pre.val==cur.val，count+=1）

点击查看代码

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def __init__(self):
        self.result = []
        self.pre = None
        self.count = 0
        self.maxCount = 0
    
    def searchBST(self, cur):
        if cur is None:
            return
        
        self.searchBST(cur.left)
        if self.pre is None:
            self.count = 1
        elif self.pre.val == cur.val:
            self.count += 1
        else:
            self.count = 1
        self.pre = cur

        if self.count == self.maxCount:
            self.result.append(cur.val)
        if self.count > self.maxCount:
            self.maxCount = self.count
            self.result = [cur.val]
        self.searchBST(cur.right)
        return

    def findMode(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        self.count = 0
        self.maxCount = 0
        self.pre = None
        self.result = []

        self.searchBST(root)
        return self.result

236.二叉树的最近公共祖先（看代码简单，但是想不通；递归；左右中：后序遍历）

点击查看代码

# 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 lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        if root == q or root == p or root is None:
            return root
        
        left = self.lowestCommonAncestor(root.left, p, q)
        right = self.lowestCommonAncestor(root.right, p, q)

        if left is not None and right is not None:
            return root
        if left is None and right is not None:
            return right
        elif left is not None and right is None:
            return left
        else:
            return None

PS:今天的后两道题没看懂，先抄一下
延迟了一天，因为昨晚有今年最大的月亮又恰逢难得的晴天，看到了月球超级靓但是还是差点意思没看到凹，不过第一次看到土星，好远好小好棒，还有环，就是没注意颜色
昨天还吃了美味寿司，一个人干了两大碟，有实力儿
破公司的秋招，把人当猴耍