Binary Tree Level Order Traversal

Iterative

/** * 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) {        List<List<Integer>> result = new ArrayList<List<Integer>>();        if (root == null) {            return result;        }        Queue<TreeNode> currentLevel = new LinkedList<TreeNode>();        Queue<TreeNode> nextLevel = new LinkedList<TreeNode>();        Queue<TreeNode> temp;        currentLevel.offer(root);        while (!currentLevel.isEmpty()) {            List<Integer> level = new ArrayList<Integer>();            while (!currentLevel.isEmpty()) {                TreeNode node = currentLevel.poll();                level.add(node.val);                if (node.left != null) {                    nextLevel.offer(node.left);                }                if (node.right != null) {                    nextLevel.offer(node.right);                }            }            result.add(level);            temp = currentLevel;            currentLevel = nextLevel;            nextLevel = temp;        }        return result;    }}