题目：
Given a binary tree, return the level order traversal of its nodes‘ values. (ie, from left to right, level by level).
For example:
Given binary tree {3,9,20,#,#,15,7},
3
/ 9 20
/ 15 7
return its level order traversal as:
[ [3], [9,20], [15,7] ]
思路：
递归
package treetraversal; import java.util.ArrayList; import java.util.List; public class BinaryTreeLevelOrderTraversal { public List<List<Integer>> levelOrder(TreeNode root) { List<List<Integer>> res = new ArrayList<List<Integer>>(); //List<Integer> record = new ArrayList<Integer>(); levelOrderTraversal(res, root, 0); return res; } private void levelOrderTraversal(List<List<Integer>> res, TreeNode root, int k) { if (root == null) return; if (res.size() < k + 1) { List<Integer> record = new ArrayList<Integer>(); record.add(root.val); res.add(record); } else { res.get(k).add(root.val); } levelOrderTraversal(res, root.left, k + 1); levelOrderTraversal(res, root.right, k + 1); } }
LeetCode - Binary Tree Level Order Traversal
原文：http://www.cnblogs.com/shuaiwhu/p/5117879.html