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

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

          1

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

          []

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

代码

class Solution {
    public List<List<Integer>> findLeaves(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        helper(res, root);
        return res;
    }

    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 level = Math.max(left, right) +1;
        if(res.size() == level){
            res.add(new ArrayList<>());
        }
        res.get(level).add(root.val);
        // 删除操作
        root.left = null;
        root.right = null;
        return level;
    }
}