題目如標題，解法都可以按照如下：


/**
* 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||root==p||root==q) return root;
TreeNode* left=lowestCommonAncestor(root->left,p,q);
TreeNode* right=lowestCommonAncestor(root->right,p,q);
if(left&&right) return root;
else if(left) return left;
else if(right) return right;
if(left==NULL&&right==NULL) return NULL;
}
};