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;
}
}