1.11 Binary Tree Level Order Traversal

Description

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] ]

Method

queue

Time and Space Complexity

o(n)

Code

public class Solution {

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

}