java-leetcode题解之第107题二叉树的层序遍历II.zip

import java.util.ArrayDeque; import java.util.ArrayList; import java.util.Deque; import java.util.LinkedList; import java.util.List; public class Solution2 { public List<List<Integer>> levelOrderBottom(TreeNode root) { List<List<Integer>> res = new ArrayList<>(); if (root == null) { return res; } Deque<List<Integer>> stack = new ArrayDeque<>(); LinkedList<TreeNode> queue = new LinkedList<>(); queue.addLast(root); while (!queue.isEmpty()) { int size = queue.size(); List<Integer> curLevel = new ArrayList<>(); for (int i = 0; i < size; i++) { TreeNode node = queue.removeFirst(); curLevel.add(node.val); if (node.left != null) { queue.addLast(node.left); } if (node.right != null) { queue.addLast(node.right); } } stack.addLast(curLevel); } while (!stack.isEmpty()) { res.add(stack.removeLast()); } return res; } }