LeetCode 102. Binary Tree Level Order Traversal

Question

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,null,null,15,7], 3 /
9 20 /
15 7 return its level order traversal as: [ [3], [9,20], [15,7] ]

Explanation

这是层级的遍历，就采用BFS即可。用Queue这个数据结构，注意每次递归时，用size来确定上层的节点个数。

Code

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