LeetCode 102. Binary Tree Level Order Traversal（二叉樹分層周遊）

原題網址：https://leetcode.com/problems/binary-tree-level-order-traversal/

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:

Given binary tree 

{3,9,20,#,#,15,7}

,

3
   / \
  9  20
    /  \
   15   7
      

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]      

方法：廣度優先搜尋。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> results = new ArrayList<>();
        if (root == null) return results;
        TreeNode[] level = new TreeNode[1];
        int size = 0;
        level[size++] = root;
        while (size > 0) {
            TreeNode[] next = new TreeNode[size * 2];
            int nextSize = 0;
            Integer[] result = new Integer[size];
            results.add(Arrays.asList(result));
            for(int i=0; i<size; i++) {
                result[i] = level[i].val;
                if (level[i].left != null) next[nextSize++] = level[i].left;
                if (level[i].right != null) next[nextSize++] = level[i].right;
            }
            level = next;
            size = nextSize;
        }
        return results;
    }
}