leetcode236 二叉树的最近公共祖先

思路：
后序遍历。
实现：
 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution
11 {
12 public:
13     TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q)
14     {
15         if (root == NULL or root == p or root == q) return root;
16         TreeNode* l = lowestCommonAncestor(root->left, p, q);
17         TreeNode* r = lowestCommonAncestor(root->right, p, q); 
18         if (l == p)
19         {
20             if (r == q) return root;
21             else return l;
22         }
23         else if (l == q)
24         {
25             if (r == p) return root;
26             else return l;
27         }
28         else if (l == NULL) return r;
29         else return l;
30     }
31 };