2017-12-11

242. Convert Binary Tree to Linked Lists by Depth 

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).

*dummy node的使用，中间node和dummy node的关系。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param root the root of binary tree
     * @return a lists of linked list
     */
    public List<ListNode> binaryTreeToLists(TreeNode root) {
        // Write your code here
        List<ListNode> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        ListNode head = new ListNode(0);
        while (!queue.isEmpty()) {
            int size = queue.size();
            ListNode ln = head;
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                ln.next = new ListNode(node.val);
                ln = ln.next;
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            res.add(head.next);
        }
        return res;
    }
}

531. Six Degrees

问题：List<UndirectedGraphNode> graph输入参数意义？

step错误，忘记return