leetcode236 二叉树的最近公共祖先

思路：

后序遍历。

实现：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
{
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q)
    {
        if (root == NULL or root == p or root == q) return root;
        TreeNode* l = lowestCommonAncestor(root->left, p, q);
        TreeNode* r = lowestCommonAncestor(root->right, p, q); 
        if (l == p)
        {
            if (r == q) return root;
            else return l;
        }
        else if (l == q)
        {
            if (r == p) return root;
            else return l;
        }
        else if (l == NULL) return r;
        else return l;
    }
};