BST的话，我们就能按照BST的定义思考了。当前遍历的node如果比我们要找的两个点都大，说明那两个点都在当前node的左边，所以这两个node的祖先肯定在当前node的左边，所以往左边找。反之，往右边找。如果这两个点一个大于当前node一个小于当前node，说明当前node就是LCA。 如果这两个点一个是另一个的祖先，那么这个点就是LCA。
1 public int query(Node t, Node u, Node v){
2 int left = u.val;
3 int right = v.val;
4 Node parent = null;
5
6 if(left > right){
7 int temp = left;
8 left = right;
9 right = temp;
10 }
11
12 while(true){
13 //t is smaller than u, v, then search in right tree
14 if(t.val < left){
15 prarent = t;
16 t = t.right;
17 }else if(t,val > right){
18 parent = t;
19 t = t.left;
20 }else if(t.val == left || t.val == right){
21 return parent.val;
22 }else{
23 return t.val;
24 }
25 }
26 }
（2）那如果不是BST呢？一般Binary Tree.
a. Tree结构中没有parent域。
1 //Worst case O(n)
2 Node getLCA(Node root, Node node1, Node node2){
3 if(root == null)
4 return null
5 if(root == node1 || root == node2)
6 return root;
7
8 Node left = getLCA(root.left, node1, node2);
9 Node right = getLCA(root.right, node1, node2);
10
11 if(left != null && right != null)
12 return root;
13 else if(left != null)
14 return left;
15 else if(right != null)
16 return right;
17 else
18 return null;
19 }
b. 如果有parent域。
1 int getHeight(Node p){
2 int height = 0;
3 while(p!=null){
4 height++;
5 p = p.parent;
6 }
7 return height;
8 }
9
10 void swap(int a, int b){
11 a = a + b;
12 b = a - b;
13 a = a - b;
14 }
15
16 Node getLCA(Node p, Node q){
17 int h1 = getHeight(p);
18 int h2 = getHeight(q);
19
20 //q is always deeper than p
21 if(h1 > h2){
22 swap(h1, h2);
23 swap(p, q);
24 }
25
26 int diff = h2 - h1;
27
28 for( int i = 0; i < diff; i++)
29 q = q.parent;
30
31 while(p!=null && q!=null){
32 //common node
33 if(p == q)
34 return p;
35 p = p.parent;
36 q = q.parent;
37 }
38
39 return NULL; //p and q are not in the same tree
40 }
