Iterative_post_order_traversal_using_2_stack.cpp

//{ Driver Code Starts
//Initial Template for C++


#include <bits/stdc++.h>
using namespace std;
#define MAX_HEIGHT 100000

// Tree Node
struct Node {
    int data;
    Node* left;
    Node* right;
};

// Utility function to create a new Tree Node
Node* newNode(int val) {
    Node* temp = new Node;
    temp->data = val;
    temp->left = NULL;
    temp->right = NULL;

    return temp;
}


// Function to Build Tree
Node* buildTree(string str) {
    // Corner Case
    if (str.length() == 0 || str[0] == 'N') return NULL;

    // Creating vector of strings from input
    // string after spliting by space
    vector<string> ip;

    istringstream iss(str);
    for (string str; iss >> str;) ip.push_back(str);

    // Create the root of the tree
    Node* root = newNode(stoi(ip[0]));

    // Push the root to the queue
    queue<Node*> queue;
    queue.push(root);

    // Starting from the second element
    int i = 1;
    while (!queue.empty() && i < ip.size()) {

        // Get and remove the front of the queue
        Node* currNode = queue.front();
        queue.pop();

        // Get the current node's value from the string
        string currVal = ip[i];

        // If the left child is not null
        if (currVal != "N") {

            // Create the left child for the current node
            currNode->left = newNode(stoi(currVal));

            // Push it to the queue
            queue.push(currNode->left);
        }

        // For the right child
        i++;
        if (i >= ip.size()) break;
        currVal = ip[i];

        // If the right child is not null
        if (currVal != "N") {

            // Create the right child for the current node
            currNode->right = newNode(stoi(currVal));

            // Push it to the queue
            queue.push(currNode->right);
        }
        i++;
    }

    return root;
}


// } Driver Code Ends
// User function Template for C++

/* Tree Node
struct Node {
    int data;
    Node* left;
    Node* right;
};*/
class Solution{
    public:
    vector<int> postOrder(Node* root) {
        
        vector<int> postorder;
        if(root == NULL) return postorder;
        
        stack<Node*> st1, st2;
        st1.push(root);
        
        while(!st1.empty())
        {
            root = st1.top();
            st1.pop();
            st2.push(root);
            if(root->left != NULL)
            {
                st1.push(root->left);
            }
            if(root->right != NULL)
            {
                st1.push(root->right);
            }
        }
        while(!st2.empty())
        {
            postorder.push_back(st2.top()->data);
            st2.pop();
        }
        return postorder;
    }
};

//{ Driver Code Starts.

/* Driver program to test size function*/

  

int main() {

   
    int t;
    scanf("%d ", &t);
    while (t--) {
        string s, ch;
        getline(cin, s);
        
        Node* root = buildTree(s);

        vector<int> ans;
        Solution ob;
        ans = ob.postOrder(root) ;

        for (int i = 0; i < ans.size(); i++)
            cout << ans[i] << " ";

        cout << endl;
     
    }
    return 0;
}

// } Driver Code Ends