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# 102. 二叉树的层次遍历 - Binary Tree Level Order Traversal

. 1 min read

## 102. 二叉树的层次遍历 - Binary Tree Level Order Traversal

【层次遍历-迭代】 【层次遍历-递归】

给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。


### Example:

    3
/ \
9  20
/  \
15   7


[
[3],
[9,20],
[15,7]
]


## Solutions

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/


### Solution 【层次遍历-迭代】 ( 2ms)

class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
List<List<Integer>> result = new LinkedList<>();
if(root == null)
return result;
queue.offer(root);
TreeNode current, left, right;
int size = 0;
List<Integer> curList;
while(queue.isEmpty() == false){
size = queue.size();
curList = new LinkedList<>();
while(size != 0){
current = queue.poll();
size--;
left = current.left;
right = current.right;
if(left != null)
if(right != null)
}
}
return result;
}
}


### Solution 【层次遍历-递归】 ( 0ms)

class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> list = new ArrayList<List<Integer>>();
function(root,list,0);
return list;
}

public void function(TreeNode node,List<List<Integer>> list,int level){
if(node == null)
return;
if(list.size()<=level)
function(node.left,list,level+1);
function(node.right,list,level+1);
}
}