[leedcode 235] Lowest Common Ancestor of a Binary Search Tree

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.

/**
* 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) {
//借助二叉搜索树的性质，左子树小于根节点，右子树大于根节点的性质，对比p，q的值跟root值的关系，可得到下面的做法
if(root==null||p==null||q==null) return null;
if(p==root||q==root) return root;
int temp=root.val;
if(temp>p.val&&temp>q.val){
return lowestCommonAncestor(root.left,p,q);
}else if(temp<p.val&&temp<q.val)
return lowestCommonAncestor(root.right,p,q);
return root;
}
}