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          

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

          1          

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

          []         

Returns [4, 5, 3], [2], [1].

 vector<vector<int>> findLeaves(TreeNode* root) {        vector<vector<int>> ret;        removeLeaves(ret, root);        return ret;    }        int removeLeaves(vector<vector<int>> & ret, TreeNode* root) {        if (root == NULL) return 0;        int d1 = removeLeaves(ret, root->left);        int d2 = removeLeaves(ret, root->right);        int lev = max(d1, d2) + 1;        if (ret.size() <= lev) ret.resize(lev);        ret[lev - 1].push_back(root->val);        return lev;    }

366. Find Leaves of Binary Tree