程式碼隨想錄演算法訓練營day18 |530.二叉搜尋樹的最小絕對差 501.二叉搜尋樹中的眾數 236. 二叉樹的最近公共祖先

Tristan241001發表於2024-10-18
演算法二叉樹

學習資料：https://programmercarl.com/0530.二叉搜尋樹的最小絕對差.html

530.二叉搜尋樹的最小絕對差（雙指標法，pre&cur，設定最小差值初始為無窮大，當差值<最小差值就更新最小差值）

點選檢視程式碼 
# 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')  # 設定最小值的初值為無窮大
        self.pre = None
    def getMinimumDifference(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return
        self.getMinimumDifference(root.left)
        # 因為中序遍歷，這裡root.val肯定大於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

501.二叉搜尋樹中的眾數（加兩個函式,init & searchBST；雙指標，當pre.val==cur.val，count+=1）

點選檢視程式碼 
# 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

236.二叉樹的最近公共祖先（看程式碼簡單，但是想不通；遞迴；左右中：後序遍歷）

點選檢視程式碼 
# 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:今天的後兩道題沒看懂，先抄一下
延遲了一天，因為昨晚有今年最大的月亮又恰逢難得的晴天，看到了月球超級靚但是還是差點意思沒看到凹，不過第一次看到土星，好遠好小好棒，還有環，就是沒注意顏色
昨天還吃了美味壽司，一個人幹了兩大碟，有實力兒
破公司的秋招，把人當猴耍

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