LeetCode 102. Binary Tree Level Order Traversal（二叉树分层遍历）

原题网址：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：广度优先搜索。

/** * 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;    }}