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[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;     } }

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