Medium

这题思路很好，通过定义一个height来添加答案.Leaf node的height定义为0， 每远离Leaf一层就height加一. 这样我们就可以通过找到每个node的height来把他们加到答案中正确的位置.

/**
 * 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<>();
        height(root, res);
        return res;    
    }
    
    private int height(TreeNode node, List<List<Integer>> res){
        if (node == null){
            return -1;
        }
        int height = 1 + Math.max(height(node.left, res), height(node.right, res));
        //height == 0, res.size() shoud be 1. if size() == 0 meaning haven't added this level
        if (res.size() == height){
            res.add(new ArrayList<>());
        }
        res.get(height).add(node.val);
        return height;
    }
}

可能不是很好看清楚recursion里面求height的同时也往res里再添加val, 画一个recursion tree方便理解具体过程.

image.png