# [leetcode] Binary Tree Postorder Traversal

Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes’ values.

For example:
Given binary tree `{1,#,2,3}`,

```   1
\
2
/
3
```

return `[3,2,1]`.

Note: Recursive solution is trivial, could you do it iteratively?

Use two stacks to simulate recursion.

stack st stores visiting nodes, stack si stores visiting values.

```//I should use a stack to simulate recursion

/**
* 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) {
//extreme case
if(root == NULL) return vector<int>();
//ordinary case
stack<TreeNode *> st;
stack<int> si;
vector<int> re;
st.push(root);
while(!st.empty()){
TreeNode * node = st.top();
st.pop();
si.push(node->val);
if(node->left) st.push(node->left); // push left first, access left last
if(node->right) st.push(node->right);// Remember, the pre-order traversal sequence is not the reverse of post-order traversal.

}
while(!si.empty()){
re.push_back(si.top());
si.pop();
}
return re;
}
};```

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