[leetcode] 236. Lowest Common Ancestor of a Binary Tree

[leetcode] 236. Lowest Common Ancestor of a Binary Tree

Description

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1Output: 3Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4Output: 5Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Note:

All of the nodes’ values will be unique.p and q are different and both values will exist in the binary tree.

分析

题目的意思是：求二叉树中两个指定结点的最小公共祖先。

首先要先确定给的两个node是否都在tree里，如果都在tree里的话，就可以分成3种情况.第一种情况是两个节点是在公共祖先的左右两侧;第二种情况是都在树的左侧;第三种情况是都在树的右侧，如果是第二，第三种情况的话，公共祖先就在给定的两个点中比较上面的那一个。

代码一（c++）

/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public: TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { if(!root||p==root||q==root){ return root; } TreeNode* left=lowestCommonAncestor(root->left,p,q); TreeNode* right=lowestCommonAncestor(root->right,p,q); if(left&&right){ return root; } return left ? left:right; }};

代码二 (python)

# Definition for a binary tree node.# class TreeNode:# def __init__(self, x):# self.val = x# self.left = None# self.right = Noneclass Solution: def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode': if(not root): return None if(root ==p or root==q): return root findLeft=self.lowestCommonAncestor(root.left,p,q) findRight=self.lowestCommonAncestor(root.right,p,q) if(not findLeft): return findRight if(not findRight): return findLeft return root

参考文献

​​[LeetCode] Lowest Common Ancestor of a Binary Tree 二叉树的最小共同父节点​​​​[LeetCode] Lowest Common Ancestor of a Binary Tree系列​​

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