107.binary-tree-level-order-traversal-ii.java

/*
 * @lc app=leetcode.cn id=107 lang=java
 * @lcpr version=30116
 *
 * [107] 二叉树的层序遍历 II
 */

// @lcpr-template-start

// @lcpr-template-end
// @lc code=start

/**
 * 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 List<List<Integer>> levelOrderBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> result = new LinkedList<>();
        if (root == null) {
            return result;
        }
        queue.add(root);
        int levelSize = 0;
        while (!queue.isEmpty()) {
            levelSize = queue.size();
            List<Integer> level = new LinkedList<>();
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            result.addFirst(level);
        }
        return result;
    }
}
// @lc code=end

/*
 * // @lcpr case=start
 * // [3,9,20,null,null,15,7]\n
 * // @lcpr case=end
 * 
 * // @lcpr case=start
 * // [1]\n
 * // @lcpr case=end
 * 
 * // @lcpr case=start
 * // []\n
 * // @lcpr case=end
 * 
 */