# Binary Tree Postorder Traversal Problem

## Description

LeetCode Problem 145.

Given the root of abinary tree, return the postorder traversal of its nodes’ values.

Example 1:

``````1
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Input: root = [1,null,2,3]
Output: [3,2,1]
``````

Example 2:

``````1
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Input: root = []
Output: []
``````

Example 3:

``````1
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Input: root = [1]
Output: [1]
``````

Example 4:

``````1
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Input: root = [1,2]
Output: [2,1]
``````

Example 5:

``````1
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Input: root = [1,null,2]
Output: [2,1]
``````

Constraints:

• The number of the nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

## Sample C++ Code

``````1
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class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
vector<int> postorder;
if (root == NULL)
return postorder;
stack<TreeNode*> s1, s2;
s1.push(root);

TreeNode* curr;
while(!s1.empty()) {
curr = s1.top();
s1.pop();
if (curr->left != NULL)
s1.push(curr->left);
if (curr->right != NULL)
s1.push(curr->right);
s2.push(curr);
}
while(!s2.empty()) {
curr = s2.top();
postorder.push_back(curr->val);
s2.pop();
}
return postorder;
}
};
``````