class Solution {
    public int countPairs(TreeNode root, int distance) {
        dfs(root,0,distance);
        return res;
    }
    private int res = 0;
    public List<Integer> dfs(TreeNode root,int level, int distance) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return list;
        if(root.left == null && root.right == null) list.add(level);

        List<Integer> left = new ArrayList<>(dfs(root.left,level+1,distance));
        List<Integer> right = new ArrayList<>(dfs(root.right,level+1,distance));
        
        for(int l : left) {
            for(int r : right) {
                if(l - level + r - level <= distance) res++;
            }
        }
        list.addAll(left);
        list.addAll(right);
        return list;
    }
}