102.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]
]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    /*
     * 使用队列
     * 每一次while循环表示一层的操作
     */
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> list = new LinkedList<List<Integer>>();
        List<TreeNode> front = new LinkedList<TreeNode>();
        
        if(root == null)
            return list;
        front.add(root);
        
        while(front.size() != 0){
            List<TreeNode> next = new LinkedList<TreeNode>();
            List<Integer> currentNum = new LinkedList<Integer>();
            
            for(TreeNode current: front){
                currentNum.add(current.val);
                
                if(current.left != null)
                    next.add(current.left);
                if(current.right != null)
                    next.add(current.right);
            }
            list.add(currentNum);
            front = next;/*交接*/
        }
        return list;
    }
}