102. 二叉树的层序遍历
很巧妙的，又学习了一种层次遍历的方法，就是说根据当前的队列的长度去遍历，遍历的当前队列的长度就是该层次的节点个数。
java
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> levelOrderList = new ArrayList<>();
if(root == null) return levelOrderList;
Queue<TreeNode> queue = new LinkedList<>(); //用于做队列，遍历整棵树
List<Integer> list = new ArrayList<>(); // 用于存储用队列遍历树的val
queue.add(root);
while(queue.size()!=0) {
// System.out.println("当前队列的长度" + queue.size());
int l = queue.size();
for(int i=0;i<l; i++) {
TreeNode front = queue.poll(); //LinkedList的poll方法可以取出第一个元素的值。
list.add(front.val);
if(front.left!=null) queue.add(front.left);
if(front.right!=null) queue.add(front.right);
}
/* System.out.print("当前放入的元素为");
Iterator iterator = list.iterator();
while(iterator.hasNext()){
System.out.print(iterator.next() + " ");
}
System.out.println(); */
levelOrderList.add(list);
list = new ArrayList<Integer>();
}
return levelOrderList;
}
}