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Check if characters of a string can be made non-decreasing by replacing ‘?’s

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  • Last Updated : 18 Jun, 2021

Given a string S of length N consisting of lowercase English alphabets and ‘?’, the task is to check if it is possible to make the string non-decreasing by replacing all the ‘?’ characters with English alphabets. If found to be true, print “Yes”. Otherwise, print “No”.

Examples:

Input: S = “abb?xy?” 
Output: Yes
Explanation:
Replacing the ‘?’s at index 3 and 6 with ‘b’ and ‘z’, modifies the string S to “abbbxyz”, which is non-decreasing.

Input: S = “??z?a?”
Output : No

Naive Approach: The simplest approach to solve the problem is to generate all possible strings using English alphabets and check if the resulting string is non-decreasing or not. If found to be true, print “Yes”, Otherwise, print “No”.

Time Complexity: O(N*26N)
Auxiliary Space: O(N)

Efficient Approach: The given problem can be solved based on the following observations: 

  • It can be observed that the only position that matters is the position with S[i]! = ‘?’, as one can replace the ‘?’ with the character at the nearest index that is not ‘?’
  • Therefore, the task is reduced to checking if characters of the strings that are not ‘?’, are in non-decreasing order or not.

Follow the steps below to solve the problem:

  • Initialize a variable, say last, to store the last character which is not a ‘?’.
  • Traverse the string and for every ith character, check if S[i]! = ‘?’ and S[i]<last, then print “No”
  • Otherwise, update last as last = S[i] if S[i]!=’?’.
  • Print “Yes” if none of the above cases are satisfied.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to check if it's possible
// to make the string s non decreasing
int nonDecreasing(string s)
{
    // Stores length of string
    int size = s.length();
 
    // Stores the last character
    // that is not '?'
    char c = 'a';
 
    // Traverse the string S
    for (int i = 0; i < size; i++) {
 
        // If current character
        // of the string is '?'
        if (s[i] == '?') {
            continue;
        }
 
        // If s[i] is not '?'
        // and is less than C
        else if ((s[i] != '?')
                 && (s[i] < c)) {
 
            return 0;
        }
 
        // Otherwise
        else {
 
            // Update C
            c = s[i];
        }
    }
 
    // Return 1
    return 1;
}
 
// Driver Code
int main()
{
    string S = "abb?xy?";
 
    if (nonDecreasing(S))
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
 
    return 0;
}

Java




// Java program for the above approach
import java.util.*;
 
class GFG
{
 
// Function to check if it's possible
// to make the String s non decreasing
static int nonDecreasing(char []s)
{
   
    // Stores length of String
    int size = s.length;
 
    // Stores the last character
    // that is not '?'
    char c = 'a';
 
    // Traverse the String S
    for (int i = 0; i < size; i++)
    {
 
        // If current character
        // of the String is '?'
        if (s[i] == '?')
        {
            continue;
        }
 
        // If s[i] is not '?'
        // and is less than C
        else if ((s[i] != '?')
                 && (s[i] < c))
        {
 
            return 0;
        }
 
        // Otherwise
        else {
 
            // Update C
            c = s[i];
        }
    }
 
    // Return 1
    return 1;
}
 
// Driver Code
public static void main(String[] args)
{
    String S = "abb?xy?";
 
    if (nonDecreasing(S.toCharArray())==1)
        System.out.print("Yes" +"\n");
    else
        System.out.print("No" +"\n");
 
}
}
 
// This code is contributed by 29AjayKumar

Python3




# python 3 program for the above approach
 
# Function to check if it's possible
# to make the string s non decreasing
def nonDecreasing(s):
   
    # Stores length of string
    size = len(s)
 
    # Stores the last character
    # that is not '?'
    c = 'a'
 
    # Traverse the string S
    for i in range(size):
       
        # If current character
        # of the string is '?'
        if (s[i] == '?'):
            continue
 
        # If s[i] is not '?'
        # and is less than C
        elif((s[i] != '?') and (s[i] < c)):
            return 0
 
        # Otherwise
        else:
           
            # Update C
            c = s[i]
 
    # Return 1
    return 1
 
# Driver Code
if __name__ == '__main__':
    S = "abb?xy?"
 
    if(nonDecreasing(S)):
        print("Yes")
    else:
        print("No")
         
        # This code is contributed by SURENDRA_GANGWAR.

C#




// C# program for the above approach
using System;
 
public class GFG
{
 
  // Function to check if it's possible
  // to make the String s non decreasing
  static int nonDecreasing(char []s)
  {
 
    // Stores length of String
    int size = s.Length;
 
    // Stores the last character
    // that is not '?'
    char c = 'a';
 
    // Traverse the String S
    for (int i = 0; i < size; i++)
    {
 
      // If current character
      // of the String is '?'
      if (s[i] == '?')
      {
        continue;
      }
 
      // If s[i] is not '?'
      // and is less than C
      else if ((s[i] != '?')
               && (s[i] < c))
      {
 
        return 0;
      }
 
      // Otherwise
      else {
 
        // Update C
        c = s[i];
      }
    }
 
    // Return 1
    return 1;
  }
 
  // Driver Code
  public static void Main(String[] args)
  {
    String S = "abb?xy?";
 
    if (nonDecreasing(S.ToCharArray())==1)
      Console.Write("Yes" +"\n");
    else
      Console.Write("No" +"\n");
 
  }
}
 
// This code is contributed by 29AjayKumar

Javascript




<script>
 
// Javascript program for the above approach
 
// Function to check if it's possible
// to make the String s non decreasing
function nonDecreasing(s)
{
     
    // Stores length of String
    let size = s.length;
 
    // Stores the last character
    // that is not '?'
    var c = 'a';
 
    // Traverse the String S
    for(let i = 0; i < size; i++)
    {
         
        // If current character
        // of the String is '?'
        if (s[i] == '?')
        {
            continue;
        }
 
        // If s[i] is not '?'
        // and is less than C
        else if ((s[i] != '?') &&
                 (s[i] < c))
        {
            return 0;
        }
 
        // Otherwise
        else
        {
             
            // Update C
            c = s[i];
        }
    }
 
    // Return 1
    return 1;
}
 
// Driver Code
let S = "abb?xy?";
 
if (nonDecreasing(S.split('')))
    document.write("Yes" + "\n");
else
    document.write("No" + "\n");
 
// This code is contributed by shivanisinghss2110
 
</script>

Output: 

Yes

 

Time Complexity: O(N)
Auxiliary Space: O(1) 


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