【Lowest Common Ancestor of a Binary Search Tree】cpp

题目：

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.

代码：

/**
* 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 ) return NULL;
if ( p->val>q->val ) swap(p, q);
if ( root->val>=p->val && root->val<=q->val )
{
return root;
}
else if ( root->val < p->val )
{
return Solution::lowestCommonAncestor(root->right, p, q);
}
else
{
return Solution::lowestCommonAncestor(root->left, p, q);
}
}
};

tips：

由于BST是数据有序的，因此找公共祖先要容易一些。

参考：http://bookshadow.com/weblog/2015/07/11/leetcode-lowest-common-ancestor-binary-search-tree/