[LeetCode] 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.

这道题不难。主要是先要搞清楚LCA的定义。

然后LCA呢只会分为三种情况，如果P,Q中最大值的那个都小于root的值，那么LCA只能在左边了。反正如果P,Q中最小值的那个都大于root的值，那么LCA肯定只能在右边。如果这两种情况都不是，那么就是最特殊的一种。LCA直接就是root。因为没有办法把这两个node一起放在一边子树去了。

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){
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);
}
return root;
}
}