Java [Leetcode 235]Lowest Common Ancestor of a Binary Search Tree

题目描述：

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.

解题思路：

对于p，q和root。如果p和q都比root小，那么LCA必定在root的左子树上面；如果p和q都比root大，那么LCA必定在右子树上面；如果一大一小，那么root即为该值。

代码如下：

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if(root == null || p == null || q == null)
return null;
if(Math.max(p.val, q.val) < root.val)
return lowestCommonAncestor(root.left, p, q);
else if(Math.min(p.val, q.val) > root.val)
return lowestCommonAncestor(root.right, p, q);
else
return root;
}
}