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 nodes2 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: