本文是在学习中的总结，欢迎转载但请注明出处：http://blog.****.net/pistolove/article/details/48392713
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.
思路：
（1）题意为给定一个BST以及该树上两节点，求这两节点的最近公共祖先。
（2）该题比较简单，通过递归很好实现。如果两节点分别分布在根节点的左右子树上，那么它们的最近公共祖先只能是树根；如果两节点同时分部在根节点的相同子树上，则需要分别递归进行判定。
（3）详情见下方代码。希望本文对你有所帮助。
算法代码实现如下：
package leetcode;
import leetcode.utils.TreeNode;
/**
*
* @author liqqc
*
*/
public class Lowest_Common_Ancestor_of_a_Binary_Search_Tree {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
// 在树的两边
if (p.val >= root.val && q.val <= root.val)
return root;
else if (p.val <= root.val && q.val >= root.val)
return root;
// 在树的一边 左边或右边
else if (p.val < root.val && q.val < root.val) {
// 左边
return lowestCommonAncestor(root.left, p, q);
}
else {
// 右边
return lowestCommonAncestor(root.right, p, q);
}
}
}