Find Leaves of Binary Tree

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.

Example:
Given binary tree

      1
     / \
    2   3
   / \
  4   5    

Returns [4, 5, 3], [2], [1].

Explanation:

1. Removing the leaves [4, 5, 3] would result in this tree:

    1
   / 

   2

2. Now removing the leaf [2] would result in this tree:

   1

3. Now removing the leaf [1] would result in the empty tree:

   []

   Returns [4, 5, 3], [2], [1].

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

         return listAll;
     }

     private TreeNode helper(List<Integer> list, TreeNode root) {
         if (root == null)
             return null;

         if (root.left == null && root.right == null) {
             list.add(root.val);
             return null;
         }

         root.left = helper(list, root.left);
         root.right = helper(list, root.right);

         return root;
     }
 }

下面这种方法是不会移除leaves的。

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public List<List<Integer>> findLeaves(TreeNode root) {
         List<List<Integer>> res = new ArrayList<>();
         height(root, res);
         return res;
     }
     private int height(TreeNode node, List<List<Integer>> res){
         if(null==node)  return ;
         int level =  + Math.max(height(node.left, res), height(node.right, res));
         if(res.size() == level - )  res.add(new ArrayList<>());
         res.get(level - ).add(node.val);
         return level;
     }
 }