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102. 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,null,null,15,7],

    3   /   9  20    /     15   7

 

return its level order traversal as:

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

思路:bfs用queue实现。
/** * 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>> res= new ArrayList<List<Integer>>();        if(root==null)        {            return res;        }                Queue<TreeNode> check=new LinkedList<TreeNode>();                check.offer(root);        while(!check.isEmpty())        {            List<Integer> curl=new ArrayList<Integer>();            for(int i=check.size()-1;i>=0;i--)            {            TreeNode find=check.poll();            curl.add(find.val);            if(find.left!=null)            {                check.offer(find.left);            }            if(find.right!=null)            {                check.offer(find.right);            }            }            res.add(curl);        }        return res;    }}

 

102. Binary Tree Level Order Traversal