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,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

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

迭代

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
   public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        levelHelper(res, root, 0);　　// 从0开始
        return res;
    }
    
    public void levelHelper(List<List<Integer>> res, TreeNode root, int height) {
        if (root == null) return;　　//说明没有子节点，结束
        if (height == res.size()) {　　//仅用于create new level 的arraylist，如果当前height小于size，那么说明此level的arraylist已经存在
            res.add(new ArrayList<Integer>());
        }
        res.get(height).add(root.val);//在对应的level add所有的value
        levelHelper(res, root.left, height+1);//下一层
        levelHelper(res, root.right, height+1);
    }
}