LC.P2415[反转二叉树的奇数层]

方法一：BFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode reverseOddLevels(TreeNode root) {
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        boolean flag = false;
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<TreeNode> list = new ArrayList<>();
            while (size-- > 0) {
                TreeNode node = queue.poll();
                if (flag) list.add(node);
                // 树为满二叉树
                if (node.left != null) {
                    queue.offer(node.left);
                    queue.offer(node.right);
                }
            }
            if (flag) {
                for (int left = 0, right = list.size() - 1; left < right; ++left, --right) {
                    int temp = list.get(left).val;
                    list.get(left).val = list.get(right).val;
                    list.get(right).val = temp;
                }
            }
            flag = !flag;
        }
        return root;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$