102.binary-tree-level-order-traversal.java

import java.util.Deque;
import java.util.LinkedList;
import java.util.Queue;

import javax.swing.tree.TreeNode;

/*
 * @lc app=leetcode id=102 lang=java
 *
 * [102] Binary Tree Level Order Traversal
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> rtn = new LinkedList<List<Integer>>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();

        if (root == null) {
            return rtn;
        }

        queue.offer(root);

        while (!queue.isEmpty()) {
            int levelTotal = queue.size();
            List<Integer> levelList = new LinkedList<Integer>();
            for (int i = 0; i < levelTotal; i++) {
                if (queue.peek().left != null) {
                    queue.offer(queue.peek().left);
                }
                if (queue.peek().right != null) {
                    queue.offer(queue.peek().right);
                }
                levelList.add(queue.poll().val);
            }
            rtn.add(levelList);
        }

        return rtn;
    }
}
// @lc code=end