Solution.java

/// Source : https://leetcode.com/problems/binary-tree-level-order-traversal/description/
/// Author : liuyubobobo
/// Time   : 2017-11-17

import java.util.ArrayList;
import java.util.List;
import java.util.LinkedList;
import javafx.util.Pair;

/// BFS
/// Time Complexity: O(n), where n is the number of nodes in the tree
/// Space Complexity: O(n)
class Solution {

    public List<List<Integer>> levelOrder(TreeNode root) {

        ArrayList<List<Integer>> res = new ArrayList<>();
        if(root == null)
            return res;

        LinkedList<Pair<TreeNode, Integer>> queue = new LinkedList<>();
        queue.addLast(new Pair<>(root, 0));

        while(!queue.isEmpty()){

            Pair<TreeNode, Integer> front = queue.removeFirst();
            TreeNode node = front.getKey();
            int level = front.getValue();

            if(level == res.size())
                res.add(new ArrayList<>());
            assert level < res.size();

            res.get(level).add(node.val);
            if(node.left != null)
                queue.addLast(new Pair<>(node.left, level + 1));
            if(node.right != null)
                queue.addLast(new Pair<>(node.right, level + 1));
        }

        return res;
    }
}