思路
从两个叶子节点往父节点找，后序遍历很适合。注意递归逻辑中的四个逻辑判断。

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (root == q || root == p || root == nullptr) return root;
        TreeNode* left = lowestCommonAncestor(root->left, p, q);
        TreeNode* right = lowestCommonAncestor(root->right, p, q);
        if (left != nullptr && right != nullptr) return root;
        if (left == nullptr && right == nullptr) return nullptr;
        if (left == nullptr && right != nullptr) return right;
        else return left;
    }
};