235. Lowest Common Ancestor of a Binary Search Tree

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题目：

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.

链接：　　http://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

题解：

BST求公共祖先。我们把两个节点的值和root进行对比，然后用二分查找的思想进行递归。

Time Complexity - O(logn)， Space Complexity - O(1)

/**
 * 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 root;
        if(p.val > root.val && q.val > root.val)
            return lowestCommonAncestor(root.right, p, q);
        else if(p.val < root.val && q.val < root.val)
            return lowestCommonAncestor(root.left, p, q);
        else
            return root;
    }
}

Reference:

http://blog.csdn.net/luckyxiaoqiang/article/details/7518888

http://zhedahht.blog.163.com/blog/static/25411174201081263815813/

https://github.com/julycoding/The-Art-Of-Programming-By-July/blob/master/ebook/zh/03.03.md

http://baozitraining.org/blog/binary-tree-common-anncestor/

http://www.cnblogs.com/felixfang/p/3828915.html

http://www.gocalf.com/blog/least-common-ancestor.html

235. Lowest Common Ancestor of a Binary Search Tree

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原文地址：http://www.cnblogs.com/yrbbest/p/5003610.html