3
       /          　5         1
   /    \      /     6       2   0    8
         /           7   4

如上圖所示，5和1的祖先是3，5和4的祖先是5；6和4的祖先是5；

 1 public class Solution {
 2     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
 3         if(root == null) return null;
 4         if
 
(root ==p || root == q) //如果root是結點p或者q，返回結點p或者q；
 5             return root;
 6         TreeNode l = lowestCommonAncestor(root.left,p,q); //返回null，說明節點p和節點q都不在root.left分支；如果l！= null，至少有一個結點在左分支
 7         TreeNode r = lowestCommonAncestor(root.right,p,q);//如果r！=null，至少有一個節點在右分支上
 8         if(l == null)
 
 9             return r;
10         if(r == null)
11             return l;
12             
13         return root;
14         
15     }
16 }

2. Lowest Common Ancestor of a Binary Search Tree

　　　　 _______6______
       /                  ___2__          ___8__
   /      \        /         0      _4       7       9
         /           3   5
代碼如下：

1 public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
2         while((root.val-p.val)*(root.val-q.val)>0){
3             root = root.val>p.val?root.left:root.right;
4         }     //就一直找到第一個root使得(root.val-p.val)*(root.val-q.val)<=0 為止

5 return root; 6 }

Lowest Common Ancestor of a Binary Search Tree