Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to thedefinition 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 allowa node to be a descendant of itself).”

        _______6______       /              /    ___2__          ___8__   /      /        /      /   0      _4       7       9         /  /         3   5

For example, the lowest common ancestor (LCA) of nodes2and8is6. Another example is LCA of nodes2and4is2, since a node can be a descendant of itself according to the LCA definition.

這道題不難。主要是先要搞清楚LCA的定義。

然后LCA呢只會分為三種情況，如果P,Q中最大值的那個都小于root的值，那么LCA只能在左邊了。反正如果P,Q中最小值的那個都大于root的值，那么LCA肯定只能在右邊。如果這兩種情況都不是，那么就是最特殊的一種。LCA直接就是root。因為沒有辦法把這兩個node一起放在一邊子樹去了。

代碼如下。~

/** * 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;    }}