时间复杂度(O( logn))
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if(root==NULL||root->val==p->val||root->val==q->val) return root;
TreeNode* node_left = lowestCommonAncestor(root->left ,p,q);
TreeNode* node_right = lowestCommonAncestor(root->right,p,q);
if(node_right==NULL) return node_left;
if(node_left==NULL ) return node_right;
return root;
}
};