题解 | 输出单层结点

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};*/
class TreeLevel {
  public:
    ListNode* getTreeLevel(TreeNode* root, int dep) {
        // write code here
        if (!root || dep < 1) return nullptr;
        if (dep == 1) return new ListNode(root->val);

        ListNode* left = getTreeLevel(root->left, dep - 1);
        ListNode* right = getTreeLevel(root->right, dep - 1);

        if (!left) return right;
        if(!right) return left;
        ListNode* tail = left;
        while (tail->next) tail = tail->next;
        tail->next = right;
        return left;
    }
};