[LeetCode] Problem 102 - Binary Tree Level Order Traversal

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Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

Example

Given binary tree [3,9,20,null,null,15,7],

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  3
 / \
9  20
  /  \
 15   7

return its level order traversal as:

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

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();

    if (root == null)
        return result;

    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);

    while (!queue.isEmpty()) {
        int size = queue.size();
        List<Integer> level = new ArrayList<>();

        for (int i = 0; i < size; i++) {
            TreeNode node = queue.poll();
            level.add(node.val);

            if (node.left != null)
                queue.offer(node.left);

            if (node.right != null)
                queue.offer(node.right);
        }

        result.add(level);
    }

    return result;
}