Given a binary tree, collect a tree's nodes as if you were doing this: Collect and remove all leaves, repeat until the tree is empty.

1.按照题目中描述的过程，一层一层把叶子节点去掉

/**
 * 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<List<Integer>>();
        if(root == null)
            return res;

        while(!isLeaf(root)) {
            List<Integer> list = new ArrayList<Integer>();
            helper(res, list, root);
            res.add(list);
        }

        List<Integer> list = new ArrayList<Integer>();
        list.add(root.val);
        res.add(list);

        return res;
    }

    public void helper(List<List<Integer>> res, List<Integer> list, TreeNode root) {
        if(root == null)
            return;
        if(isLeaf(root.left)) {
            list.add(root.left.val);
            root.left = null;
        } else {
            helper(res, list, root.left);
        }

        if(isLeaf(root.right)) {
            list.add(root.right.val);
            root.right = null;
        } else {
            helper(res, list, root.right);
        }
    }

    public boolean isLeaf(TreeNode root) {
        if(root == null)
            return false;
        if(root.left == null && root.right == null)
            return true;

        return false;
    }
}

2.更讲道理的做法，求每个节点的深度，深度定义为左右子节点深度最大值+1，把深度作为index

/**
 * 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;
    }

    public 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 depth = Math.max(left, right) + 1;
        while(depth >= res.size())
            res.add(new ArrayList<Integer>());

        res.get(depth).add(root.val);
        return depth;
    }
}