GFG_Tree_ReverseOrderTraversal.cpp

/* https://practice.geeksforgeeks.org/problems/reverse-level-order-traversal/1#
 * Given a binary tree of size N, find its reverse level order traversal. ie- the traversal must begin from the last level.
 */

// { Driver Code Starts
#include <bits/stdc++.h>
using namespace std;

struct Node
{
    int data;
    struct Node *left;
    struct 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;
}

vector<int> reverseLevelOrder(Node* root);

int main()
{

    int t;
    scanf("%d ",&t);
    while(t--)
    {
        string s;
        getline(cin,s);
        Node* root = buildTree(s);
        vector<int> result = reverseLevelOrder(root);
        for (int i = 0; i < result.size(); ++i)
        {
            cout<<result[i]<<" ";
        }
        cout<<endl;
    }
    return 1;
}// } Driver Code Ends


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

vector<int> reverseLevelOrder(Node *root)
{
    vector<int> rlo;

    queue<Node *> q;
    if(root)q.push(root);

    while(!q.empty()){

        Node *t = q.front(); q.pop();

        rlo.push_back(t->data);

        if(t->right) q.push(t->right);
        if(t->left) q.push(t->left);
    }//while q

    reverse(rlo.begin(), rlo.end());

    return rlo;
}

//Using List Container
vector<int> reverseLevelOrder1(Node *root)
{
    list<int> rlo;

    queue<Node *> q;
    if(root)q.push(root);

    while(!q.empty()){

        Node *t = q.front(); q.pop();

        rlo.push_front(t->data);

        if(t->right) q.push(t->right);
        if(t->left) q.push(t->left);
    }//while q

    vector<int> rlo_ret(rlo.begin(), rlo.end());
    return rlo_ret;
}