Leaf node's out degree is 0.

Like remove outsize notes of graph.

Not simple as counting heights but still O(n) time and space

```
class Solution {
void helper(TreeNode* cur) {
if (!cur)
return;
if (cur->left) {
graph[cur->left] = cur;
outdegree[cur]++;
}
if (cur->right) {
graph[cur->right] = cur;
outdegree[cur]++;
}
if (!cur->left && !cur->right)
outdegree[cur] = 0;
helper(cur->left);
helper(cur->right);
}
unordered_map<TreeNode*, TreeNode*> graph;
unordered_map<TreeNode*, int> outdegree;
public:
vector<vector<int>> findLeaves(TreeNode* root) {
helper(root);
vector<vector<int>> res;
while (outdegree.size()) {
cout << outdegree.size() << endl;
vector<int> level;
vector<TreeNode*> rem;
for (auto i : outdegree)
if (i.second == 0) {
level.push_back(i.first->val);
rem.push_back(i.first);
}
for (auto i : rem) {
if (graph.find(i) != graph.end())
outdegree[graph[i]]--;
outdegree.erase(i);
}
res.push_back(level);
}
return res;
}
};
```