LeetCode 235. Lowest Common Ancestor of a Binary Search Tree

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.

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

题意即寻找一个二叉搜索树中两个节点最近的共同祖先。

我们可以从BST的特性下手，当某个节点的数值恰好在给定两个节点数值的闭区间内，那么该节点就是最近祖先。代码如下：

class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if (!p || !q || !root) return NULL;
int maxer = max(p->val,q->val),miner = min(p->val,q->val);
if (maxer >= root->val && miner <= root->val)
return root;
else if (root->val > maxer)
return lowestCommonAncestor(root->left,p,q);
else
return lowestCommonAncestor(root->right,p,q);
}
};