Convert Ternary Expression to a Binary Tree

Given a string that contains ternary expressions. The expressions may be nested, task is convert the given ternary expression to a binary Tree. 

Examples:

Input :  string expression =   a?b:c 
Output :        a
              /  \
             b    c

Input : expression =  a?b?c:d:e
Output :     a
           /  \
          b    e
        /  \
       c    d

Asked In : Facebook Interview

Idea is that we traverse a string make first character as root and do following step recursively . 

1. If we see Symbol ‘?’ 
   • then we add next character as the left child of root. 
2. If we see Symbol ‘:’ 
   • then we add it as the right child of current root. 

do this process until we traverse all element of “String”. 

Below is the implementation of above idea  

C++

// C++ program to convert a ternary expression to
// a tree.
#include<bits/stdc++.h>
using namespace std;

// tree structure
struct Node
{
    char data;
    Node *left, *right;
};

// function create a new node
Node *newNode(char Data)
{
    Node *new_node = new Node;
    new_node->data = Data;
    new_node->left = new_node->right = NULL;
    return new_node;
}

// Function to convert Ternary Expression to a Binary
// Tree. It return the root of tree
// Notice that we pass index i by reference because we
// want to skip the characters in the subtree
Node *convertExpression(string str, int & i)
{
    // store current character of expression_string
    // [ 'a' to 'z']
    Node * root =newNode(str[i]);

    //If it was last character return
    //Base Case
    if(i==str.length()-1) return root;

    // Move ahead in str
    i++;
    //If the next character is '?'.Then there will be subtree for the current node
    if(str[i]=='?')
    {
        //skip the '?'
        i++;

        // construct the left subtree
        // Notice after the below recursive call i will point to ':'
        // just before the right child of current node since we pass i by reference
        root->left = convertExpression(str,i);

        //skip the ':' character
        i++;

        //construct the right subtree
        root->right = convertExpression(str,i);
        return root;
    }
    //If the next character is not '?' no subtree just return it
    else return root;
}

// function print tree
void printTree( Node *root)
{
    if (!root)
        return ;
    cout << root->data <<" ";
    printTree(root->left);
    printTree(root->right);
}

// Driver program to test above function
int main()
{
    string expression = "a?b?c:d:e";
    int i=0;
    Node *root = convertExpression(expression, i);
    printTree(root) ;
    return 0;
}

Java

// Java program to convert a ternary
// expression to a tree.
import java.util.Queue;
import java.util.LinkedList;

// Class to represent Tree node
class Node
{
    char data;
    Node left, right;

    public Node(char item)
    {
        data = item;
        left = null;
        right = null;
    }
}

// Class to convert a ternary expression to a Tree
class BinaryTree
{
    // Function to convert Ternary Expression to a Binary
    // Tree. It return the root of tree
    Node convertExpression(char[] expression, int i)
    {
        // Base case
        if (i >= expression.length)
            return null;

        // store current character of expression_string
        // [ 'a' to 'z']
        Node root = new Node(expression[i]);

        // Move ahead in str
        ++i;

        // if current character of ternary expression is '?'
        // then we add next character as a left child of
        // current node
        if (i < expression.length && expression[i]=='?')
            root.left = convertExpression(expression, i+1);

        // else we have to add it as a right child of
        // current node expression.at(0) == ':'
        else if (i < expression.length)
            root.right = convertExpression(expression, i+1);

        return root;
    }

    // function print tree
    public void printTree( Node root)
    {
        if (root == null)
            return;

        System.out.print(root.data +" ");
        printTree(root.left);
        printTree(root.right);
    }

// Driver program to test above function
    public static void main(String args[])
    {
        String exp = "a?b?c:d:e";
        BinaryTree tree = new BinaryTree();
        char[] expression=exp.toCharArray();
        Node root = tree.convertExpression(expression, 0);
        tree.printTree(root) ;
    }
}

/* This code is contributed by Mr. Somesh Awasthi */

Python3

# Class to define a node
# structure of the tree
class Node:
    def __init__(self, key):
        self.data = key
        self.left = None
        self.right = None

# Function to convert ternary
# expression to a Binary tree
# It returns the root node
# of the tree
def convert_expression(expression, i):
    if i >= len(expression):
        return None

    # Create a new node object
    # for the expression at
    # ith index
    root = Node(expression[i])

    i += 1

    # if current character of
    # ternary expression is '?'
    # then we add next character
    # as a left child of
    # current node
    if (i < len(expression) and
                expression[i] is "?"):
        root.left = convert_expression(expression, i + 1)

    # else we have to add it
    # as a right child of
    # current node expression[0] == ':'
    elif i < len(expression):
        root.right = convert_expression(expression, i + 1)
    return root

# Function to print the tree
# in a pre-order traversal pattern
def print_tree(root):
    if not root:
        return
    print(root.data, end=' ')
    print_tree(root.left)
    print_tree(root.right)

# Driver Code
if __name__ == "__main__":
    string_expression = "a?b?c:d:e"
    root_node = convert_expression(string_expression, 0)
    print_tree(root_node)

# This code is contributed
# by Kanav Malhotra

C#

// C# program to convert a ternary
// expression to a tree.
using System;

// Class to represent Tree node
public class Node
{
    public char data;
    public Node left, right;

    public Node(char item)
    {
        data = item;
        left = null;
        right = null;
    }
}

// Class to convert a ternary
// expression to a Tree
public class BinaryTree
{
    // Function to convert Ternary Expression
    // to a Binary Tree. It return the root of tree
    public virtual Node convertExpression(char[] expression,
                                          int i)
    {
        // Base case
        if (i >= expression.Length)
        {
            return null;
        }

        // store current character of
        // expression_string [ 'a' to 'z']
        Node root = new Node(expression[i]);

        // Move ahead in str
        ++i;

        // if current character of ternary expression
        // is '?' then we add next character as a
        // left child of current node
        if (i < expression.Length && expression[i] == '?')
        {
            root.left = convertExpression(expression, i + 1);
        }

        // else we have to add it as a right child
        // of current node expression.at(0) == ':'
        else if (i < expression.Length)
        {
            root.right = convertExpression(expression, i + 1);
        }

        return root;
    }

    // function print tree
    public virtual void printTree(Node root)
    {
        if (root == null)
        {
            return;
        }

        Console.Write(root.data + " ");
        printTree(root.left);
        printTree(root.right);
    }

// Driver Code
public static void Main(string[] args)
{
    string exp = "a?b?c:d:e";
    BinaryTree tree = new BinaryTree();
    char[] expression = exp.ToCharArray();
    Node root = tree.convertExpression(expression, 0);
    tree.printTree(root);
}
}

// This code is contributed by Shrikant13

Javascript

<script>

// Javascript program to convert a ternary
// expreesion to a tree.

// Class to represent Tree node
class Node
{
    constructor(item)
    {
        this.data = item;
        this.left = null;
        this.right = null;
    }
}

// Function to convert Ternary Expression
// to a Binary Tree. It return the root of tree
function convertExpression(expression, i)
{

    // Base case
    if (i >= expression.length)
    {
        return null;
    }

    // Store current character of
    // expression_string [ 'a' to 'z']
    var root = new Node(expression[i]);

    // Move ahead in str
    ++i;

    // If current character of ternary expression
    // is '?' then we add next character as a
    // left child of current node
    if (i < expression.length && expression[i] == '?')
    {
        root.left = convertExpression(expression, i + 1);
    }

    // Else we have to add it as a right child
    // of current node expression.at(0) == ':'
    else if (i < expression.length)
    {
        root.right = convertExpression(expression, i + 1);
    }
    return root;
}

// Function print tree
function printTree(root)
{
    if (root == null)
    {
        return;
    }
    document.write(root.data + " ");
    printTree(root.left);
    printTree(root.right);
}

// Driver code
var exp = "a?b?c:d:e";
var expression = exp.split('');
var root = convertExpression(expression, 0);
printTree(root);

// This code is contributed by noob2000

</script>

Output

a b c d e 

Time Complexity : O(n) [ here n is length of String ]
Auxiliary Space: O(n)

Approach for Converting Ternary Expression to Binary Tree.

• The algorithm uses a recursive approach to build the tree in a top-down manner.
• It starts with creating a new node for the current character at the current index.
• If the next character is a ‘?’, it means that the current node needs a left child. So, the algorithm calls itself recursively with the next index to create the left child of the current node.
• If the next character is a ‘:’, it means that the current node needs a right child. So, the algorithm calls itself recursively with the next index to create the right child of the current node.
• Finally, the algorithm returns the root node of the binary tree.

Here is the implementation of above approach:-

C++

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

class Node {
public:
    char val;
    Node *left, *right;
    Node(char v) : val(v), left(nullptr), right(nullptr) {}
};

Node* ternaryToTree(string exp) {
    if (exp.empty()) return nullptr;
    Node *root = new Node(exp[0]);
    stack<Node*> st;
    st.push(root);
    for (int i = 1; i < exp.size(); i += 2) {
        Node *cur = st.top(); st.pop();
        if (exp[i] == '?') {
            cur->left = new Node(exp[i+1]);
            st.push(cur);
            st.push(cur->left);
        } else {
            cur->right = new Node(exp[i+1]);
            st.push(cur->right);
        }
    }
    return root;
}

void printTree(Node* root) {
    if (!root) return;
    cout << root->val << " ";
    printTree(root->left);
    printTree(root->right);
}

int main() {
    string exp = "a?b?c:d:e";
    Node *root = ternaryToTree(exp);
    printTree(root);
    return 0;
}

Java

import java.util.*;

class Node {
    char val;
    Node left, right;
    public Node(char v) { val = v; }
}

class Solution {
    public static Node ternaryToTree(String exp) {
        if (exp == null || exp.length() == 0) return null;
        Node root = new Node(exp.charAt(0));
        Stack<Node> st = new Stack<>();
        st.push(root);
        for (int i = 1; i < exp.length(); i += 2) {
            Node cur = st.pop();
            if (exp.charAt(i) == '?') {
                cur.left = new Node(exp.charAt(i+1));
                st.push(cur);
                st.push(cur.left);
            } else {
                cur.right = new Node(exp.charAt(i+1));
                st.push(cur.right);
            }
        }
        return root;
    }

    public static void printTree(Node root) {
        if (root == null) return;
        System.out.print(root.val + " ");
        printTree(root.left);
        printTree(root.right);
    }

    public static void main(String[] args) {
        String exp = "a?b?c:d:e";
        Node root = ternaryToTree(exp);
        printTree(root);
    }
}

Python3

class Node:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

def ternary_to_tree(exp):
    if not exp:
        return None
    root = Node(exp[0])
    stack = [root]
    for i in range(1, len(exp)):
        if exp[i] == '?':
            stack[-1].left = Node(exp[i+1])
            stack.append(stack[-1].left)
        elif exp[i] == ':':
            stack.pop()
            while stack[-1].right:
                stack.pop()
            stack[-1].right = Node(exp[i+1])
            stack.append(stack[-1].right)
    return root

def print_tree(root):
    if not root:
        return
    print(root.val, end=' ')
    print_tree(root.left)
    print_tree(root.right)

if __name__ == "__main__":
    exp = "a?b?c:d:e"
    root = ternary_to_tree(exp)
    print_tree(root)

Javascript

class Node {
  constructor(val) {
    this.val = val;
    this.left = null;
    this.right = null;
  }
}

let i = 0;

function convertExpression(expression) {
  if (!expression || i >= expression.length) {
    return null;
  }

  const root = new Node(expression[i]);
  i++;

  if (i < expression.length && expression[i] === "?") {
    i++;
    root.left = convertExpression(expression);
  }

  if (i < expression.length && expression[i] === ":") {
    i++;
    root.right = convertExpression(expression);
  }

  return root;
}

function printTree(root) {
  if (!root) {
    return;
  }

  console.log(root.val + " ");
  printTree(root.left);
  printTree(root.right);
}

const stringExpression = "a?b?c:d:e";
const rootNode = convertExpression(stringExpression);
printTree(rootNode);

Output

a b c d e 

Time complexity: O(n) – Since we visit each character of the expression exactly once.

Space complexity: O(n) – Since in the worst case, the recursion stack can grow to the height of the tree, which can be O(n) if the ternary expression is a degenerate tree (a long chain of ‘?’). Additionally, we store O(n) nodes in the binary tree.

This article is contributed by Nishant Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.