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;
    }
}