/**
 * 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* find_node(TreeNode* cur, TreeNode* p, TreeNode* q)
    {
      //发现目标点或者空点返回
        if(cur==NULL || cur->val == p->val || cur->val == q->val) return cur;
        //当前值大于目标点，则目标点在左子树
        if(cur->val > p->val && cur->val > q->val) 
        {
            TreeNode * left_tree = find_node(cur->left,p,q);
            return left_tree;
        }
        //当前值小于目标点，则目标点在右子树
        else if (cur->val < p->val && cur->val < q->val) 
        {
            TreeNode * right_tree = find_node(cur->right,p,q);
            return right_tree;
        }
        //当前值在两个目标点中间，则当前值是公共祖先返回
        else 
        {
           return cur;
        }
    }
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        return find_node(root,p,q);
    }
};

/**
 * 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* result;
    int track_back(TreeNode* cur, TreeNode* p, TreeNode* q)
    {
        if(cur == NULL) return 0;
        int flag = 0;
        TreeNode *left_node;
        TreeNode *right_node;
        if(p->val <= q->val)
        {
            left_node = p;
            right_node = q;
        }else
        {
            left_node = q;
            right_node = p;
        }
        if(cur->val >= left_node->val && cur->val <= right_node->val )
        {
            flag += track_back(cur->left,p,q);
            flag += track_back(cur->right,p,q);
        }else if(cur->val < left_node->val)
            flag += track_back(cur->right,p,q);
        else if(cur->val > right_node->val)
            flag += track_back(cur->left,p,q);
        if(cur == left_node) flag += 1;
        else if(cur == right_node) flag +=2;
        if(flag == 3 )
        {
             result = cur;
             return 0;
        }
        return flag;
    }
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        track_back(root,p,q);
        return result;
    }
};