Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level)

BinaryTreeLevelOrderTraversal.java

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> out = new ArrayList<List<Integer>>();
        dfs(root, out, -1);
        return out;
    }
    
    public void dfs(TreeNode node, List<List<Integer>> out, int dep) {
        dep++;
        if(node != null) {
            visit(node, out, dep);
            dfs(node.left, out, dep);
            dfs(node.right, out, dep);
        }
    }
    
    public void visit(TreeNode node, List<List<Integer>> out, int dep) {
        if (out.size() <= dep){
             out.add(new ArrayList<Integer>());
        }
        out.get(dep).add(node.val);
    }
}