[Leetcode] Binary Tree Level Order Traversal

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,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

Solution 1: BFS

/**
 * Definition for binary tree
 * 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>> ret=new ArrayList<List<Integer>>();
        if(root==null)
            return ret;
        
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        List<Integer> al=new ArrayList<Integer>();
        
        queue.add(root);
        int curLvl=1;
        int nexLvl=0;
        
        while(!queue.isEmpty()){
            TreeNode cur=queue.remove();
            al.add(cur.val);
            curLvl--;
            
            if(cur.left!=null){
                queue.add(cur.left);
                nexLvl++;
            }
            if(cur.right!=null){
                queue.add(cur.right);
                nexLvl++;
            }
            if(curLvl==0){
                ret.add(al);
                al=new ArrayList<Integer>();
                curLvl=nexLvl;
                nexLvl=0;
            }
        }
        
        return ret;
    }
}

Solution 2: DFS

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    List<List<Integer>> ret;
    
    public List<List<Integer>> levelOrder(TreeNode root) {
        ret = new ArrayList<List<Integer>>();
        goDeeper(0, root);
        return ret;
    }

    private void goDeeper(int level, TreeNode root) {
        // TODO Auto-generated method stub
        if (root == null)
            return;
        if (ret.size() > level) {
            List<Integer> a = ret.get(level);
            a.add(root.val);
        } else {
            List<Integer> a = new LinkedList<Integer>();
            a.add(root.val);
            ret.add(a);
        }
        goDeeper(level+1, root.left);
        goDeeper(level+1, root.right);
    }
}