Given a binary tree, design an algorithm which creates a linked list of all the nodes at each depth (e.g., if you have a tree with depth D, you'll have D linked lists).

Have you met this question in a real interview?

Yes

Example

Given binary tree:

    1
   / \
  2   3
 /
4

return

[
  1-
>
null,
  2-
>
3-
>
null,
  4-
>
null
]

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    /**
     * @param root the root of binary tree
     * @return a lists of linked list
     */
    vector<ListNode*> binaryTreeToLists(TreeNode* root) {
        // Write your code here
        vector<ListNode*> results;
        if (root == NULL) {
            return results;
        }

        queue<TreeNode*> q;
        q.push(root);

        while (!q.empty()) {
            int size = q.size();
            ListNode head(0);
            ListNode* pre = &head;
            for (int i = 0; i < size; ++i) {
                TreeNode* cur = q.front(); q.pop();
                pre->next = new ListNode(cur->val);
                pre = pre->next;
                if (cur->left) {
                    q.push(cur->left);
                }
                if (cur->right) {
                    q.push(cur->right);
                }
            }
            results.push_back(head.next);
        }

        return results;
    }
};