LeetCode——Binary Tree Level Order Traversal

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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]]

原题链接：https://oj.leetcode.com/problems/binary-tree-level-order-traversal/

题目：给定一个二叉树，返回节点的层序遍历的值。（如，从左到右，一层一层）

思路：类似于广度优先遍历，可以将节点依次放入到队列中，再按层取出。

public List<List<Integer>> levelOrder(TreeNode root) {List<List<Integer>> list = new ArrayList<List<Integer>>();if (root == null)return list;Queue<TreeNode> queue = new LinkedList<TreeNode>();queue.add(root);while (!queue.isEmpty()) {List<Integer> li = new ArrayList<Integer>();int size = queue.size();for (int i = 0; i < size; i++) {TreeNode node = queue.poll();li.add(node.val);if (node.left != null)queue.add(node.left);if (node.right != null)queue.add(node.right);}list.add(li);}return list;}// Definition for binary treepublic class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int x) {val = x;}}

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