LeetCode #235

# Description:

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

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).”

## Example:

``````        _______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.
``````

# Code:

``````class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if(root==p || root==q) return root; // p or q is the ancestor

int min_val = min(p->val, q->val);
int max_val = max(p->val, q->val);

// 根据BST的性质，那就该是root
if(root->val>min_val && root->val<max_val) return root;

// root比两个都小，那应该在right subtree
if(root->val < min_val) return lowestCommonAncestor(root->right, p, q);

//root比两个都大，那应该在left subtree
if(root->val > max_val) return lowestCommonAncestor(root->left, p, q);
}
};
``````