The intuition is very straightforward, postorder traversal is the reverse of the "root->right->left" preorder traversal.

```
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
vector<int> ret;
stack<TreeNode*> s;
if(root == nullptr)
return ret;
s.push(root);
TreeNode* temp;
while(!s.empty()){
temp = s.top();
s.pop();
ret.push_back(temp->val);
if(temp->left)
s.push(temp->left);
if(temp->right)
s.push(temp->right);
}
reverse(ret.begin(),ret.end());
return ret;
}
};
```