剑指 Offer 32 - I. 从上到下打印二叉树

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层序遍历

  1. class Solution {
  2.     public int[] levelOrder(TreeNode root) {
  3.         if (root == null) return new int[0];
  5.         Queue<TreeNode> queue = new LinkedList<>();
  6.         queue.add(root);
  7.         List<Integer> list = new ArrayList<>();
  9.         while (!queue.isEmpty()) {
  10.             for (int i = queue.size(); i > 0; i--) {
  11.                 TreeNode cur = queue.poll();
  12.                 list.add(cur.val);
  14.                 if (cur.left != null) queue.add(cur.left);
  15.                 if (cur.right != null) queue.add(cur.right);
  16.             }
  17.         }
  18.         return list.stream().mapToInt(Integer::intValue).toArray();
  19.     }
  20. }