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
    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
    3
      1
     /
    2

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

    1
    1

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

    1
    []

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

其实就是把具有相同height的节点放在一起。

代码如下：

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