Medium
这道题终于在第五次做的时候，明白了Binary Tree的Height和Depth的区别，Height是从下往上数，Leaf的height为0， Root的height最大；而Depth是从上往下数，root的depth为0， Leaf的depth最高。所以这道题说是Find leaves, 其实是把所有node按Height分，比如第一层所有的Leaf nodes就是height = 0的node; 第二层的Leaf nodes就是height = 1的node; 第三层的leaf nodes就是height = 3的nodes。 这样发现这个问题其实只和height有关，就很好解决了。

/**
 * 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>> findLeaves(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null){
            return res;
        }
        helper(res, root);
        return res;
    }
    //搞清楚树的height和depth的区别
    private int helper(List<List<Integer>> res, TreeNode root){
        if (root == null){
            return -1;
        }            
        int left = helper(res, root.left);
        int right = helper(res, root.right);
        int height = Math.max(left, right) + 1;
        //if we have height 0, size should be 1; if we have height 1, size should be 2. 
        //so if we do have each level in res, height should be one less than size.
        //However if height == size, meaning we don't have that level yet.
        if (height == res.size()){
            res.add(new ArrayList<Integer>());
        }
        res.get(height).add(root.val);
        return height;
    }
}