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,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its level order traversal as:
[
[3],
[9,20],
[15,7]
]
输出一棵树的每一行，很简单。
/**
* 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 = new ArrayList<List<Integer>>(); if( root == null)
return list;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
while( !queue.isEmpty() ){
List ans = new ArrayList<Integer>();
int size = queue.size();
for( int i = 0;i<size;i++){
TreeNode node = queue.poll();
if( node == null)
continue;
ans.add(node.val);
queue.add(node.left);
queue.add(node.right);
}
if( ans.size() != 0)
list.add(ans); } return list;
}
}