Find Leaves of Binary Tree

Given a binary tree, find all leaves and then remove those leaves. Then repeat the previous steps until the tree is empty.

Example: Given binary tree 1 / \ 2 3 / \
4 5
Returns [4, 5, 3], [2], [1].

Explanation:

1. Remove the leaves [4, 5, 3] from the tree

      1
     / 
    2          
   

2. Remove the leaf [2] from the tree

      1          
   

3. Remove the leaf [1] from the tree

      []         
   

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

public class Solution { private List> res = new ArrayList<>();

public List<List<Integer>> findLeaves(TreeNode root) {
    helper(root);
    return res;
}

private int helper(TreeNode node){
    if(null==node){
        return -1;
    }
    int level = 1 + Math.max(helper(node.left), helper(node.right));
    if(res.size()<level+1){
        res.add(new ArrayList<Integer>());
    }
    res.get(level).add(node.val);
    return level;
}

}