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).”
即在一个BST中，找到两个节点p和q的最小公共祖先。
思路：
由于是BST，因此可以分别求出p和q的查找路径pPath,qPath
找出pPath和qPath的第一个不同节点即可。
实现代码：
/**
* Definition for a binary tree node.
* public class TreeNode {
* public int val;
* public TreeNode left;
* public TreeNode right;
* public TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
var pathP = new List<TreeNode>();
Path(root, p, pathP);
var pathQ = new List<TreeNode>();
Path(root, q , pathQ);
var i = 0;
while(i < pathP.Count || i < pathQ.Count){
if(i >= pathP.Count ||i >= pathQ.Count){
return pathP[--i];
}
if(pathP[i].val == pathQ[i].val){
i ++;
}
else{
return pathP[--i];
}
}
return null;
}
public void Path(TreeNode node, TreeNode n, IList<TreeNode> list){
if(node == null){
return ;
}
list.Add(node);
if(n.val == node.val){
return;
}
if(n.val < node.val){
Path(node.left, n ,list);
}
else{
Path(node.right, n ,list);
}
}
}