Binary Tree Level Order Traversal

Given a binary tree, return thelevel ordertraversal 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]
]

图例

分析

代码

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        List<List<Integer>> result = new ArrayList<>();
        if(root ==null) return result;
        q.add(root);
        while(!q.isEmpty()){
            List<Integer> currLevel = new ArrayList<>();
            int levelNum = q.size();
            for(int i = 0; i<levelNum; i++){
                TreeNode node = q.poll();
                currLevel.add(node.val);
                if(node.left!=null){
                    q.add(node.left);
                }
                if(node.right!=null){
                    q.add(node.right);
                }
            } 
            result.add(currLevel) ;
        }
       return result; 

    }
}