70. 두 갈래 나무의 차원 훑어보기 II

        3
       / \
      9  20
        /  \
       15   7

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

코드

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: buttom-up level order a list of lists of integer
     */
    public List> levelOrderBottom(TreeNode root) {
        List> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue queue = new LinkedList();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            List level = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode head = queue.poll();
                level.add(head.val);
                if (head.left != null) {
                    queue.offer(head.left);
                }
                if (head.right != null) {
                    queue.offer(head.right);
                }
            }
            result.add(level);
        }
        
        //  ， O(n)
        Collections.reverse(result);
        return result;
    }
}