/**
 * 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 == nullptr)
            return root;
        if (root == p || root == q)
            return root;
        TreeNode* l = lowestCommonAncestor (root -> left , p , q);
        TreeNode* r = lowestCommonAncestor ( root -> right , p , q);

        if (l && r)
            return root;
        if ( l && !r)
            return l;
        else return r;

    }
};