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] ]
层序遍历
/**
* 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>> re = new LinkedList<List<Integer>>();
if (root == null) {
return re;
}
ArrayDeque<TreeNode> queue = new ArrayDeque<TreeNode>();
TreeNode tmp;
int size = 0;
queue.offer(root);
while (!queue.isEmpty()) {
List<Integer> l = new LinkedList<Integer>();
size = queue.size();
for (int i = 0; i < size; i++) {
tmp = queue.poll();
if (tmp.left != null) {
queue.offer(tmp.left);
}
if (tmp.right != null) {
queue.offer(tmp.right);
}
l.add(tmp.val);
}
re.add(l);
}
return re;
}
}