算法作业HW30：LeetCode 235. Lowest Common Ancestor of a Binary Search Tree

Description:

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

 

Note:

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

_______6______
/              \
___2__          ___8__
/      \        /      \
0      _4       7       9
/  \
3   5

For example, the lowest common ancestor (LCA) of nodes 

2

 and 

8

 is 

6

.
Another example is LCA of nodes

2

 and 

4

 is 

2

,
since a node can be a descendant of itself according to the LCA definition.

Solution:

  Analysis and Thinking:

题目要求对于给定的一个二叉搜索树，找出给出的两个结点的最近公共祖先，可以使用递归的思想实现

  Steps:

1. 如果当前两待搜索结点都小于根结点，进行右子树递归

2. 如果当前两待搜索结点都大于根结点，进行左子树递归

3. 如果两节点大小不等（一个大于根结点，一个小于），返回根结点

Codes:

class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if (p->val > root->val && q->val > root->val)
return lowestCommonAncestor(root->right, p, q);

if (p->val < root->val && q->val < root->val)
return lowestCommonAncestor(root->left, p, q);

return root;
}
};

Results: