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],
return its level order traversal as:
1 2 3 4 5
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[ [3], [9,20], [15,7] ]
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Code
1 2 3 4 5 6
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public class { int val; TreeNode left; TreeNode right; TreeNode(int x) { val = x; } }
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
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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; }
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