Largest number not exceeding N that does not contain any of the digits of S

Difficulty Level : Hard
Last Updated : 12 Jan, 2021

Given a numberic string N (1 ≤ |N| ≤ 10⁵) and another numeric string S (1 ≤ |S| ≤ 10⁵), the task is to find the maximum number ≤ N such that it doesn’t contain digits of string S. Remove leading zeros, if required.
Note: The string S doesn’t contain ‘0’.

Examples:

Input: N = “2004”, S = “2”
Output: 1999
Explanation:
2004 has digit 2 which is in the string S.
Therefore, keep reducing it by 1 until the number obtained does not contain 2.
Therefore, the largest number that can be obtained is 1999.
Input: N = “12345”, S = “23”
Output: 11999

Naive Approach: For every value of N, check if it contains any of the digits of S. Keep reducing N by 1 until any digit that is present in S does not occur in N.
Time Complexity: O(|N| * log(|N|) * |S|)
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized using Hashing. Follow the steps below to solve the problem:

Iterate over the characters of the string S and store the distinct characters of S in a Map.
Traverse the string N.
If any digit of N is found to be present in S, say idx^th digit, reduce N[idx] to the largest digit not present in S, say LargestDig.
Digits in range [idx + 1, |N| – 1] are changed to LargestDig.

Illustration:
Consider N = “2004” and S = “2”.
To remove 2, it is changed to 1 and the remaining digits are changed to the largest digit which is not present in S, that is digit 9.

Remove all the leading zeros from the number.

Below is the implementation of the above approach.

C++

// C++ program to implement
// the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to obtain the largest number not exceeding
// num which does not contain any digits present in S
string greatestReducedNumber(string num, string s)
{
    // Stores digits of S
    vector<bool> vis_s(10, false);
 
    // Traverse the string S
    for (int i = 0; i < (int)s.size(); i++) {
 
        // Set occurrence as true
        vis_s[int(s[i]) - 48] = true;
    }
 
    int n = num.size();
    int in = -1;
 
    // Traverse the string n
    for (int i = 0; i < n; i++) {
        if (vis_s[(int)num[i] - '0']) {
            in = i;
            break;
        }
    }
 
    // All digits of num are not
    // present in string s
    if (in == -1) {
        return num;
    }
 
    for (char dig = num[in]; dig >= '0'; dig--) {
        if (vis_s[(int)dig - '0'] == 0) {
            num[in] = dig;
            break;
        }
    }
 
    char LargestDig = '0';
 
    for (char dig = '9'; dig >= '0'; dig--) {
        if (vis_s[dig - '0'] == false) {
 
            // Largest Digit not present
            // in the string s
            LargestDig = dig;
            break;
        }
    }
 
    // Set all digits from positions
    // in + 1 to n - 1 as LargestDig
    for (int i = in + 1; i < n; i++) {
        num[i] = LargestDig;
    }
 
    // Counting leading zeroes
    int Count = 0;
    for (int i = 0; i < n; i++) {
        if (num[i] == '0')
            Count++;
        else
            break;
    }
 
    // Removing leading zeroes
    num.erase(0, Count);
 
    // If the string becomes null
    if ((int)num.size() == 0)
        return "0";
 
    // Return the largest number
    return num;
}
 
// Driver Code
int main()
{
    string N = "12345";
    string S = "23";
 
    cout << greatestReducedNumber(N, S);
 
    return 0;
}

Java

// Java program to implement
// the above approach
 
import java.io.*;
import java.util.Arrays;
 
class GFG {
 
    // Function to obtain the largest number not exceeding
    // num which does not contain any digits present in S
    static String greatestReducedNumber(String num,
                                        String s)
    {
        // Stores digits of S
        Boolean[] vis_s = new Boolean[10];
        Arrays.fill(vis_s, Boolean.FALSE);
 
        // Traverse the string S
        for (int i = 0; i < (int)s.length(); i++) {
 
            // Set occurrence as true
            vis_s[(int)(s.charAt(i)) - 48] = true;
        }
 
        int n = num.length();
        int in = -1;
 
        // Traverse the string n
        for (int i = 0; i < n; i++) {
            if (vis_s[(int)num.charAt(i) - '0']) {
                in = i;
                break;
            }
        }
 
        // All digits of num are not
        // present in string s
        if (in == -1) {
            return num;
        }
 
    for (char dig = num.charAt(in); dig >= '0'; dig--) {
        if (vis_s[(int)dig - '0'] == false) {
            num = num.substring(0, in) + dig
                  + num.substring(in + 1, n);
            break;
        }
    }
 
    char LargestDig = '0';
 
    for (char dig = '9'; dig >= '0'; dig--) {
        if (vis_s[dig - '0'] == false) {
 
            // Largest Digit not present
            // in the string s
            LargestDig = dig;
            break;
        }
    }
 
    // Set all digits from positions
    // in + 1 to n - 1 as LargestDig
    for (int i = in + 1; i < n; i++) {
        num = num.substring(0, i) + LargestDig;
    }
 
    // Counting leading zeroes
    int Count = 0;
    for (int i = 0; i < n; i++) {
        if (num.charAt(i) == '0')
            Count++;
        else
            break;
    }
 
    // Removing leading zeroes
    num = num.substring(Count, n);
 
    // If the string becomes null
    if ((int)num.length() == 0)
        return "0";
 
    // Return the largest number
    return num;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        String N = "12345";
        String S = "23";
 
        System.out.print(greatestReducedNumber(N, S));
    }
}
 
// This code is contributed by subhammahato348

C#

// C# program to implement
// the above approach
using System;
using System.Collections.Generic;   
class GFG 
{
 
  // Function to obtain the largest number not exceeding
  // num which does not contain any digits present in S
  static string greatestReducedNumber(string num, string s)
  {
     
    // Stores digits of S
    bool[] vis_s = new bool[10];
 
    // Traverse the string S
    for (int i = 0; i < (int)s.Length; i++) {
 
      // Set occurrence as true
      vis_s[(int)(s[i]) - 48] = true;
    }
 
    int n = num.Length;
    int In = -1;
 
    // Traverse the string n
    for (int i = 0; i < n; i++) {
      if (vis_s[(int)num[i] - '0']) {
        In = i;
        break;
      }
    }
 
    // All digits of num are not
    // present in string s
    if (In == -1) {
      return num;
    }
 
    for (char dig = num[In]; dig >= '0'; dig--) {
      if (vis_s[(int)dig - '0'] == false) {
        num = num.Substring(0, In) + dig + num.Substring(In + 1, n - In - 1);
        break;
      }
    }
 
    char LargestDig = '0';
 
    for (char dig = '9'; dig >= '0'; dig--) {
      if (vis_s[dig - '0'] == false) {
 
        // Largest Digit not present
        // in the string s
        LargestDig = dig;
        break;
      }
    }
 
    // Set all digits from positions
    // in + 1 to n - 1 as LargestDig
    for (int i = In + 1; i < n; i++) {
      num = num.Substring(0, i) + LargestDig;
    }
 
    // Counting leading zeroes
    int Count = 0;
    for (int i = 0; i < n; i++) {
      if (num[i] == '0')
        Count++;
      else
        break;
    }
 
    // Removing leading zeroes
    num = num.Substring(Count, n);
 
    // If the string becomes null
    if ((int)num.Length == 0)
      return "0";
 
    // Return the largest number
    return num;
  }
 
  // Driver code
  static void Main() {
    string N = "12345";
    string S = "23";
 
    Console.Write(greatestReducedNumber(N, S));
  }
}
 
// This code is contibuted by divyesh072019

Output:

Time complexity: O(|N| + |s|)
Auxiliary space: O(1)

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