102. Binary Tree Level Order Traversal

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LeetCode

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Problem

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]
]

Code

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root == null) return new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        List<List<Integer>> res = new ArrayList<>();
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            for(int i = 0; i < size; i++){
                TreeNode curr = queue.poll();
                level.add(curr.val);
                if(curr.left != null) queue.offer(curr.left);
                if(curr.right != null) queue.offer(curr.right);
            }

            res.add(level);
        }

        return res;
    }
}