Check if a binary tree is sorted level-wise or not

Given a binary tree. The task is to check if the binary tree is sorted level-wise or not. A binary tree is level sorted if max( i-1^th level) is less than min( i^th level ).

Examples:

Input :        1 
              / \
             /   \
            2     3
           / \   / \
          /   \ /   \
         4    5 6    7
Output : Sorted

Input:         1 
              / 
             4 
            / \
           6   5
                \
                 2
Output: Not sorted

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Simple Solution: A simple solution is to compare minimum and maximum value of each adjacent level i and i+1. Traverse to i^th and i+1^th level, compare the minimum value of i+1^th level with maximum value of i^th level and return the result.

Time complexity: O(n²).

Efficient Solution: An efficient solution is to do level order traversal and keep track of the minimum and maximum values of current level. Use a variable prevMax to store the maximum value of the previous level. Then compare the minimum value of current level with the maximum value of the previous level, pevMax. If minimum value is greater than the prevMax, then the given tree is sorted level-wise up to current level. For next level, prevMax is the equal to maximum value of current level. So update the prevMax with maximum value of current level. Repeat this until all levels of given tree are not traversed.

Below is the implementation of above approach:

C++

// CPP program to determine whether 
// binary tree is level sorted or not. 
  
#include <bits/stdc++.h> 
using namespace std; 
  
// Structure of a tree node. 
struct Node { 
    int key; 
    Node *left, *right; 
}; 
  
// Function to create new tree node. 
Node* newNode(int key) 
{ 
    Node* temp = new Node; 
    temp->key = key; 
    temp->left = temp->right = NULL; 
    return temp; 
} 
  
// Function to determine if 
// given binary tree is level sorted 
// or not. 
int isSorted(Node* root) 
{ 
  
    // to store maximum value of previous 
    // level. 
    int prevMax = INT_MIN; 
  
    // to store minimum value of current 
    // level. 
    int minval; 
  
    // to store maximum value of current 
    // level. 
    int maxval; 
  
    // to store number of nodes in current 
    // level. 
    int levelSize; 
  
    // queue to perform level order traversal. 
    queue<Node*> q; 
    q.push(root); 
  
    while (!q.empty()) { 
  
        // find number of nodes in current 
        // level. 
        levelSize = q.size(); 
  
        minval = INT_MAX; 
        maxval = INT_MIN; 
  
        // traverse current level and find 
        // minimum and maximum value of 
        // this level. 
        while (levelSize > 0) { 
            root = q.front(); 
            q.pop(); 
  
            levelSize--; 
  
            minval = min(minval, root->key); 
            maxval = max(maxval, root->key); 
  
            if (root->left) 
                q.push(root->left); 
  
            if (root->right) 
                q.push(root->right); 
        } 
  
        // if minimum value of this level 
        // is not greater than maximum 
        // value of previous level then 
        // given tree is not level sorted. 
        if (minval <= prevMax) 
            return 0; 
  
        // maximum value of this level is 
        // previous maximum value for 
        // next level. 
        prevMax = maxval; 
    } 
  
    return 1; 
} 
  
// Driver program 
int main() 
{ 
    /* 
            1  
           /  
          4    
           \ 
            6 
           / \ 
          8   9 
         /     \ 
        12     10 
    */
  
    Node* root = newNode(1); 
    root->left = newNode(4); 
    root->left->right = newNode(6); 
    root->left->right->left = newNode(8); 
    root->left->right->right = newNode(9); 
    root->left->right->left->left = newNode(12); 
    root->left->right->right->right = newNode(10); 
  
    if (isSorted(root)) 
        cout << "Sorted"; 
    else
        cout << "Not sorted"; 
    return 0; 
} 

Java

// Java program to determine whether  
// binary tree is level sorted or not.  
import java.util.*; 
  
class GfG {  
  
// Structure of a tree node.  
static class Node {  
    int key;  
    Node left, right;  
} 
  
// Function to create new tree node.  
static Node newNode(int key)  
{  
    Node temp = new Node();  
    temp.key = key;  
    temp.left = null; 
    temp.right = null;  
    return temp;  
}  
  
// Function to determine if  
// given binary tree is level sorted  
// or not.  
static int isSorted(Node root)  
{  
  
    // to store maximum value of previous  
    // level.  
    int prevMax = Integer.MIN_VALUE;  
  
    // to store minimum value of current  
    // level.  
    int minval;  
  
    // to store maximum value of current  
    // level.  
    int maxval;  
  
    // to store number of nodes in current  
    // level.  
    int levelSize;  
  
    // queue to perform level order traversal.  
    Queue<Node> q = new LinkedList<Node> ();  
    q.add(root);  
  
    while (!q.isEmpty()) {  
  
        // find number of nodes in current  
        // level.  
        levelSize = q.size();  
  
        minval = Integer.MAX_VALUE;  
        maxval = Integer.MIN_VALUE;  
  
        // traverse current level and find  
        // minimum and maximum value of  
        // this level.  
        while (levelSize > 0) {  
            root = q.peek();  
            q.remove();  
  
            levelSize--;  
  
            minval = Math.min(minval, root.key);  
            maxval = Math.max(maxval, root.key);  
  
            if (root.left != null)  
                q.add(root.left);  
  
            if (root.right != null)  
                q.add(root.right);  
        }  
  
        // if minimum value of this level  
        // is not greater than maximum  
        // value of previous level then  
        // given tree is not level sorted.  
        if (minval <= prevMax)  
            return 0;  
  
        // maximum value of this level is  
        // previous maximum value for  
        // next level.  
        prevMax = maxval;  
    }  
  
    return 1;  
}  
  
// Driver program  
public static void main(String[] args)  
{  
    /*  
            1  
        /  
        4  
        \  
            6  
        / \  
        8 9  
        /     \  
        12     10  
    */
  
    Node root = newNode(1);  
    root.left = newNode(4);  
    root.left.right = newNode(6);  
    root.left.right.left = newNode(8);  
    root.left.right.right = newNode(9);  
    root.left.right.left.left = newNode(12);  
    root.left.right.right.right = newNode(10);  
  
    if (isSorted(root) == 1)  
        System.out.println("Sorted");  
    else
        System.out.println("Not sorted");  
} 
}  

Python3

# Python3 program to determine whether  
# binary tree is level sorted or not.  
from queue import Queue 
  
# Function to create new tree node.  
class newNode: 
    def __init__(self, key):  
        self.key = key  
        self.left = self.right = None
  
# Function to determine if given binary 
# tree is level sorted or not.  
def isSorted(root): 
  
    # to store maximum value of previous  
    # level.  
    prevMax = -999999999999
  
    # to store minimum value of current  
    # level.  
    minval = None
  
    # to store maximum value of current  
    # level.  
    maxval = None
  
    # to store number of nodes in current  
    # level.  
    levelSize = None
  
    # queue to perform level order traversal.  
    q = Queue() 
    q.put(root)  
  
    while (not q.empty()):  
  
        # find number of nodes in current  
        # level.  
        levelSize = q.qsize()  
  
        minval = 999999999999
        maxval = -999999999999
  
        # traverse current level and find  
        # minimum and maximum value of  
        # this level.  
        while (levelSize > 0):  
            root = q.queue[0]  
            q.get()  
  
            levelSize -= 1
  
            minval = min(minval, root.key)  
            maxval = max(maxval, root.key)  
  
            if (root.left): 
                q.put(root.left)  
  
            if (root.right):  
                q.put(root.right) 
  
        # if minimum value of this level  
        # is not greater than maximum  
        # value of previous level then  
        # given tree is not level sorted.  
        if (minval <= prevMax):  
            return 0
  
        # maximum value of this level is  
        # previous maximum value for  
        # next level.  
        prevMax = maxval 
    return 1
  
# Driver Code  
if __name__ == '__main__': 
      
    #  
    #     1  
    #     /  
    #     4  
    #     \  
    #     6  
    #     / \  
    #     8 9  
    #     /     \  
    # 12     10  
    root = newNode(1)  
    root.left = newNode(4)  
    root.left.right = newNode(6)  
    root.left.right.left = newNode(8)  
    root.left.right.right = newNode(9)  
    root.left.right.left.left = newNode(12)  
    root.left.right.right.right = newNode(10)  
  
    if (isSorted(root)):  
        print("Sorted")  
    else: 
        print("Not sorted") 
  
# This code is contributed by PranchalK 

C#

// C# program to determine whether  
// binary tree is level sorted or not.  
using System; 
using System.Collections.Generic;  
      
class GFG  
{  
  
// Structure of a tree node.  
public class Node 
{  
    public int key;  
    public Node left, right;  
} 
  
// Function to create new tree node.  
static Node newNode(int key)  
{  
    Node temp = new Node();  
    temp.key = key;  
    temp.left = null; 
    temp.right = null;  
    return temp;  
}  
  
// Function to determine if  
// given binary tree is level sorted  
// or not.  
static int isSorted(Node root)  
{  
  
    // to store maximum value of previous  
    // level.  
    int prevMax = int.MinValue;  
  
    // to store minimum value of current  
    // level.  
    int minval;  
  
    // to store maximum value of current  
    // level.  
    int maxval;  
  
    // to store number of nodes in current  
    // level.  
    int levelSize;  
  
    // queue to perform level order traversal.  
    Queue<Node> q = new Queue<Node> ();  
    q.Enqueue(root);  
  
    while (q.Count != 0)  
    {  
  
        // find number of nodes in current  
        // level.  
        levelSize = q.Count;  
  
        minval = int.MaxValue;  
        maxval = int.MinValue;  
  
        // traverse current level and find  
        // minimum and maximum value of  
        // this level.  
        while (levelSize > 0) 
        {  
            root = q.Peek();  
            q.Dequeue();  
  
            levelSize--;  
  
            minval = Math.Min(minval, root.key);  
            maxval = Math.Max(maxval, root.key);  
  
            if (root.left != null)  
                q.Enqueue(root.left);  
  
            if (root.right != null)  
                q.Enqueue(root.right);  
        }  
  
        // if minimum value of this level  
        // is not greater than maximum  
        // value of previous level then  
        // given tree is not level sorted.  
        if (minval <= prevMax)  
            return 0;  
  
        // maximum value of this level is  
        // previous maximum value for  
        // next level.  
        prevMax = maxval;  
    }  
  
    return 1;  
}  
  
// Driver Code 
public static void Main(String[] args)  
{  
    /*  
            1  
        /  
        4  
        \  
            6  
        / \  
        8 9  
        /     \  
        12     10  
    */
    Node root = newNode(1);  
    root.left = newNode(4);  
    root.left.right = newNode(6);  
    root.left.right.left = newNode(8);  
    root.left.right.right = newNode(9);  
    root.left.right.left.left = newNode(12);  
    root.left.right.right.right = newNode(10);  
  
    if (isSorted(root) == 1)  
        Console.WriteLine("Sorted");  
    else
        Console.WriteLine("Not sorted");  
} 
}  
  
// This code is contributed by PrinciRaj1992  

Output:

Sorted

Time Complexity: O(n)
Auxiliary Space: O(n)

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My Personal Notes

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	18
2019	2020	2021

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

C#

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