Consecutive sequenced numbers in a string

Given a string that contains only numeric digits, we need to check whether that strings contains numbers in a consecutive sequential manner in increasing order. 

Note: Negative numbers are not considered part of this problem. So we consider that input only contains positive integer.

Examples: 

Input :  str = "1234"
Output : Yes 
         1
Explanation : 
There are 1, 2, 3, 4 which are 
consecutive and in increasing order.
And the starting number is 1

Input :  str = "91012"
Output : No
Explanation : 
There are no such sequence in the
string. 

Input :  str = "99100"
Output : Yes 
         99
Explanation : The consecutive sequential 
numbers are 99, 100

Input :  str = "010203"
Output : NO 
Explanation : 
Although at first glance there seems to
be 01, 02, 03. But those wouldn't be 
considered a number. 01 is not 1  it's 0, 1 

Approach: An easily implementable and useful approach is to start taking one character at first (assuming that our string starts with 1 digit number) and then form a new string by concatenating the next number until the length of new string is equal to original string. 

Perhaps an example may clarify : 

Let’s take string “99100”

Below is the implementation:

C++

// CPP Program to check whether a string contains
// consecutive sequential numbers or not
#include <iostream>
using namespace std;

// function to check consecutive sequential number
int isConsecutive(string str)
{
    // variable to store starting number
    int start;

    // length of the input string
    int length = str.size();

    // find the number till half of the string
    for (int i = 0; i < length / 2; i++) {

        // new string containing the starting
        // substring of input string
        string new_str = str.substr(0, i + 1);

        // converting starting substring into number
        int num = atoi(new_str.c_str());

        // backing up the starting number in start
        start = num;

        // while loop until the new_string is
        // smaller than input string
        while (new_str.size() < length) {

            // next number
            num++;

            // concatenate the next number
            new_str += to_string(num);
        }

        // check if new string becomes equal to
        // input string
        if (new_str == str)
           return start;
    }

    // if string doesn't contains consecutive numbers
    return -1;
}

// Driver's Code
int main()
{
    string str = "99100";
    cout << "String: " << str << endl;
    int start = isConsecutive(str);
    if (start != -1)
        cout << "Yes \n" << start << endl;
    else
        cout << "No" << endl;

    string str1 = "121315";
    cout << "\nString: " << str1 << endl;
    start = isConsecutive(str1);
    if (start != -1)
        cout << "Yes \n" << start << endl;
    else
        cout << "No" << endl;

    return 0;
}

Java

// Java Program to check whether a String contains
// consecutive sequential numbers or not
class GFG
{

// function to check consecutive sequential number
static int isConsecutive(String str)
{
    // variable to store starting number
    int start;

    // length of the input String
    int length = str.length();

    // find the number till half of the String
    for (int i = 0; i < length / 2; i++)
    {

        // new String containing the starting
        // substring of input String
        String new_str = str.substring(0, i + 1);

        // converting starting substring into number
        int num = Integer.parseInt(new_str);

        // backing up the starting number in start
        start = num;

        // while loop until the new_String is
        // smaller than input String
        while (new_str.length() < length)
        {

            // next number
            num++;

            // concatenate the next number
            new_str = new_str + String.valueOf(num);
        }

        // check if new String becomes equal to
        // input String
        if (new_str.equals(str))
        return start;
    }

    // if String doesn't contains consecutive numbers
    return -1;
}

// Driver Code
public static void main(String[] args)
{
    String str = "99100";
    System.out.println("String: " + str);
    int start = isConsecutive(str);
    if (start != -1)
        System.out.println("Yes \n" + start);
    else
        System.out.println("No");

    String str1 = "121315";
    System.out.println("\nString: " + str1);
    start = isConsecutive(str1);
    if (start != -1)
        System.out.println("Yes \n" + start);
    else
        System.out.println("No");
    }
}

// This code contributed by Rajput-Ji

Python3

# Python Program to check whether a String contains
# consecutive sequential numbers or not

# function to check consecutive sequential number
def isConsecutive(strs):

    # variable to store starting number
    start = 0;

    # length of the input String
    length = len(strs);

    # find the number till half of the String
    for i in range(length // 2):

        # new String containing the starting
        # substring of input String
        new_str = strs[0: i + 1];

        # converting starting substring into number
        num = int(new_str);

        # backing up the starting number in start
        start = num;

        # while loop until the new_String is
        # smaller than input String
        while (len(new_str) < length):
            # next number
            num += 1;

            # concatenate the next number
            new_str = new_str + str(num);

        # check if new String becomes equal to
        # input String
        if (new_str==(strs)):
            return start;

    # if String doesn't contains consecutive numbers
    return -1;

# Driver Code
if __name__ == '__main__':
    str0 = "99100";
    print("String: " + str0);
    start = isConsecutive(str0);
    if (start != -1):
        print("Yes \n" , start);
    else:
        print("No");

    str1 = "121315";
    print("\nString: " , str1);
    start = isConsecutive(str1);
    if (start != -1):
        print("Yes \n" , start);
    else:
        print("No");

# This code is contributed by shikhasingrajput

C#

// C# Program to check whether a String contains
// consecutive sequential numbers or not
using System;

class GFG
{

// function to check consecutive sequential number
static int isConsecutive(String str)
{
    // variable to store starting number
    int start;

    // length of the input String
    int length = str.Length;

    // find the number till half of the String
    for (int i = 0; i < length / 2; i++)
    {

        // new String containing the starting
        // substring of input String
        String new_str = str.Substring(0, i + 1);

        // converting starting substring into number
        int num = int.Parse(new_str);

        // backing up the starting number in start
        start = num;

        // while loop until the new_String is
        // smaller than input String
        while (new_str.Length < length)
        {

            // next number
            num++;

            // concatenate the next number
            new_str = new_str + String.Join("",num);
        }

        // check if new String becomes equal to
        // input String
        if (new_str.Equals(str))
        return start;
    }

    // if String doesn't contains consecutive numbers
    return -1;
}

// Driver Code
public static void Main(String[] args)
{
    String str = "99100";
    Console.WriteLine("String: " + str);
    int start = isConsecutive(str);
    if (start != -1)
        Console.WriteLine("Yes \n" + start);
    else
        Console.WriteLine("No");

    String str1 = "121315";
    Console.WriteLine("\nString: " + str1);
    start = isConsecutive(str1);
    if (start != -1)
        Console.WriteLine("Yes \n" + start);
    else
        Console.WriteLine("No");
    }
}

// This code has been contributed by 29AjayKumar

Javascript

<script>

// JavaScript Program to check whether a String contains
// consecutive sequential numbers or not

// function to check consecutive sequential number
function isConsecutive(str)
{
    // variable to store starting number
    let start;

    // length of the input String
    let length = str.length;

    // find the number till half of the String
    for (let i = 0; i < length / 2; i++)
    {

        // new String containing the starting
        // substring of input String
        let new_str = str.substring(0, i + 1);

        // converting starting substring into number
        let num = parseInt(new_str);

        // backing up the starting number in start
        start = num;

        // while loop until the new_String is
        // smaller than input String
        while (new_str.length < length)
        {

            // next number
            num++;

            // concatenate the next number
            new_str = new_str + (num).toString();
        }

        // check if new String becomes equal to
        // input String
        if (new_str == (str))
        return start;
    }

    // if String doesn't contains consecutive numbers
    return -1;
}

// Driver Code
let str = "99100";
document.write("String: " + str+"<br>");
let start = isConsecutive(str);
if (start != -1)
    document.write("Yes <br>" + start+"<br>");
else
    document.write("No<br>");

let str1 = "121315";
document.write("<br>String: " + str1+"<br>");
start = isConsecutive(str1);
if (start != -1)
    document.write("Yes <br>" + start+"<br>");
else
    document.write("No<br>");



// This code is contributed by rag2127

</script>

Output

String: 99100
Yes 
99

String: 121315
No

Time Complexity: O(n²), where n is the size of the given string.
Auxiliary Space: O(n), where n is the size of the given string.