categories: [Blog,Algorithm]

107. 二叉树的层序遍历 II

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode() {}
  8.  *     TreeNode(int val) { this.val = val; }
  9.  *     TreeNode(int val, TreeNode left, TreeNode right) {
  10.  *         this.val = val;
  11.  *         this.left = left;
  12.  *         this.right = right;
  13.  *     }
  14.  * }
  15.  */
  16. class Solution {
  17.     public List<List<Integer>> levelOrderBottom(TreeNode root) {
  18.         List<List<Integer>> levelOrder = new LinkedList<List<Integer>>();
  19.         if (root == null) {
  20.             return levelOrder;
  21.         }
  22.         Queue<TreeNode> queue = new LinkedList<TreeNode>();
  23.         queue.offer(root);
  24.         while (!queue.isEmpty()) {
  25.             List<Integer> level = new ArrayList<Integer>();
  26.             int size = queue.size();
  27.             for (int i = 0; i < size; i++) {
  28.                 TreeNode node = queue.poll();
  29.                 level.add(node.val);
  30.                 TreeNode left = node.left, right = node.right;
  31.                 if (left != null) {
  32.                     queue.offer(left);
  33.                 }
  34.                 if (right != null) {
  35.                     queue.offer(right);
  36.                 }
  37.             }
  38.             levelOrder.add(0, level);//最后添加的在最开始位置
  39.         }
  40.         return levelOrder;
  42. // 作者：LeetCode-Solution
  43. // 链接：https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/solution/er-cha-shu-de-ceng-ci-bian-li-ii-by-leetcode-solut/
  44. // 来源：力扣（LeetCode）
  45. // 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
  46.     }
  47. }