leetcode366. Find Leaves of Binary Tree(递归)

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:

Input: [1,2,3,4,5]

      1
     / \
    2   3
   / \     
  4   5    

Output: [[4,5,3],[2],[1]]

/**
 * 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<>(); 
        help(root,res);
        return res;
    }
    public int help(TreeNode root, List<List<Integer>> res){
        if(root==null) return -1;
        
        int level=1+Math.max(help(root.left,res),help(root.right,res));
        
        if(res.size()<level+1)  res.add(new ArrayList<>());
        res.get(level).add(root.val);
                
        return level;
    }  
}