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# Replace ‘?’ in a string such that no two adjacent characters are same

• Difficulty Level : Easy
• Last Updated : 16 Jul, 2021

Given a string S of length N consisting of “?” and lowercase letters, the task is to replace “?” with lowercase letters such that no adjacent characters are the same. If more than one possible combination exists, print any one of them.

Examples:

Input: S = “?a?a”
Output: baba
Explanation:
Replacing all ‘?’s with ‘b’ modifies the string to “baba”.
Since no adjacent characters in “baba” are the same, print the string as the answer.

Input: S = “???”
Output: aca
Explanation:
Replace first ‘?’ with ‘a’.
Replace second ‘?’ with ‘c’.
Replace third ‘?’ with ‘a’. Now, the modified string is “aca”.
Therefore, there are no adjacent characters in “ca” which are same.

Naive Approach: The simplest approach is to try generating all possible permutations of the given string consisting of lowercase letters. There can be 26N strings. In each of these strings, check whether adjacent characters matches or not and all lowercase characters in the given string matches the chosen permutation of the string.

Time Complexity: O(N*26N), where N is the length of the given string.
Auxiliary Space: O(N)

Efficient Approach: To optimize the above approach, the idea is to replace every ‘?’ by the character ‘a’ and check if this character is equal to the adjacent character or not. If it is equal to the adjacent character then increment the current character. Below are the steps:

1. If the first character of the string is ‘?’ then replace it with ‘a’ and if it is equal to the next character then increment the current character by 1
2. Traverse the given string using a variable i over the range [1, N – 1] and if the current character is ‘?’ and do the following:
• Update character at index i as s[i] = ‘a’.
• Now if the character at index i and (i – 1) are the same then increment the current character by 1.
• Now if the character at index i and (i + 1) are the same then increment the current character by 1.
• Now if the character at index i and (i – 1) are the same again, then increment the current character by 1. This step is mandatory because after increment character in the above step it might be possible character at index i and  (i – 1) are the same.
3. If the last character of the string is ‘?’ then replace it with ‘a’ and if it is equal to the previous character then increment the last character by 1
4. Print the string after the above steps.

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach` `#include "bits/stdc++.h"``using` `namespace` `std;` `// Function that replace all '?' with``// lowercase alphabets such that each``// adjacent character is different``string changeString(string S)``{``    ``// Store the given string``    ``string s = S;` `    ``int` `N = (``int``)s.length();` `    ``// If the first character is '?'``    ``if` `(s == ``'?'``) {``        ``s = ``'a'``;``        ``if` `(s == s) {``            ``s++;``        ``}``    ``}` `    ``// Traverse the string [1, N - 1]``    ``for` `(``int` `i = 1; i < N - 1; i++) {` `        ``// If the current character is '?'``        ``if` `(s[i] == ``'?'``) {` `            ``// Change the character``            ``s[i] = ``'a'``;` `            ``// Check equality with``            ``// the previous character``            ``if` `(s[i] == s[i - 1]) {``                ``s[i]++;``            ``}` `            ``// Check equality with``            ``// the next character``            ``if` `(s[i] == s[i + 1]) {``                ``s[i]++;``            ``}` `            ``// Check equality with``            ``// the previous character``            ``if` `(s[i] == s[i - 1]) {``                ``s[i]++;``            ``}``        ``}``    ``}` `    ``// If the last character is '?'``    ``if` `(s[N - 1] == ``'?'``) {` `        ``// Change character``        ``s[N - 1] = ``'a'``;` `        ``// Check with previous character``        ``if` `(s[N - 1] == s[N - 2]) {``            ``s[N - 1]++;``        ``}``    ``}` `    ``// Return the resultant string``    ``return` `s;``}` `// Driver Code``int` `main()``{``    ``// Given string S``    ``string S = ``"?a?a"``;` `    ``// Function Call``    ``cout << changeString(S);` `    ``return` `0;``}`

## Java

 `// Java program for``// the above approach``class` `GFG{` `// Function that replace all '?' with``// lowercase alphabets such that each``// adjacent character is different``static` `String changeString(String S)``{``  ``// Store the given String``  ``char` `[]s = S.toCharArray();` `  ``int` `N = (``int``)S.length();` `  ``// If the first character is '?'``  ``if` `(s[``0``] == ``'?'``)``  ``{``    ``s[``0``] = ``'a'``;``    ``if` `(s[``0``] == s[``1``])``    ``{``      ``s[``0``]++;``    ``}``  ``}` `  ``// Traverse the String [1, N - 1]``  ``for` `(``int` `i = ``1``; i < N - ``1``; i++)``  ``{``    ``// If the current``    ``// character is '?'``    ``if` `(s[i] == ``'?'``)``    ``{``      ``// Change the character``      ``s[i] = ``'a'``;` `      ``// Check equality with``      ``// the previous character``      ``if` `(s[i] == s[i - ``1``])``      ``{``        ``s[i]++;``      ``}` `      ``// Check equality with``      ``// the next character``      ``if` `(s[i] == s[i + ``1``])``      ``{``        ``s[i]++;``      ``}` `      ``// Check equality with``      ``// the previous character``      ``if` `(s[i] == s[i - ``1``])``      ``{``        ``s[i]++;``      ``}``    ``}``  ``}` `  ``// If the last character is '?'``  ``if` `(s[N - ``1``] == ``'?'``)``  ``{``    ``// Change character``    ``s[N - ``1``] = ``'a'``;` `    ``// Check with previous``    ``// character``    ``if` `(s[N - ``1``] == s[N - ``2``])``    ``{``      ``s[N - ``1``]++;``    ``}``  ``}` `  ``String ans = ``""``;``  ` `  ``for``(``int`  `i = ``0``; i < s.length; i++)``  ``{``    ``ans += s[i];``  ``}``  ` `  ``// Return the resultant String``  ``return` `ans;``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``  ``// Given String S``  ``String S = ``"?a?a"``;` `  ``// Function Call``  ``System.out.print(changeString(S));``}``}` `// This code is contributed by Rajput-Ji`

## Python3

 `# Python3 program for``# the above approach` `# Function that replace all '?' with``# lowercase alphabets such that each``# adjacent character is different``def` `changeString(S):``    ` `    ``# Store the given String``    ``N ``=` `len``(S)``    ``s ``=` `[``' '``] ``*` `(``len``(S))``    ` `    ``for` `i ``in` `range``(``len``(S)):``        ``s[i] ``=` `S[i]` `    ``# If the first character is '?'``    ``if` `(s[``0``] ``=``=` `'?'``):``        ``s[``0``] ``=` `'a'``        ` `        ``if` `(s[``0``] ``=``=` `s[``1``]):``            ``s[``0``] ``=` `chr``(``ord``(s[``0``]) ``+` `1``)` `    ``# Traverse the String [1, N - 1]``    ``for` `i ``in` `range``(``1``, N ``-` `1``):``        ` `        ``# If the current``        ``# character is '?'``        ``if` `(s[i] ``=``=` `'?'``):``            ` `            ``# Change the character``            ``s[i] ``=` `'a'` `            ``# Check equality with``            ``# the previous character``            ``if` `(s[i] ``=``=` `s[i ``-` `1``]):``                ``s[i] ``=`  `chr``(``ord``(s[i]) ``+` `1``)` `            ``# Check equality with``            ``# the next character``            ``if` `(s[i] ``=``=` `s[i ``+` `1``]):``                ``s[i] ``=`  `chr``(``ord``(s[i]) ``+` `1``)` `            ``# Check equality with``            ``# the previous character``            ``if` `(s[i] ``=``=` `s[i ``-` `1``]):``                ``s[i] ``=`  `chr``(``ord``(s[i]) ``+` `1``)` `    ``# If the last character is '?'``    ``if` `(s[N ``-` `1``] ``=``=` `'?'``):``        ` `        ``# Change character``        ``s[N ``-` `1``] ``=` `'a'``        ` `        ``# Check with previous``        ``# character``        ``if` `(s[N ``-` `1``] ``=``=` `s[N ``-` `2``]):``            ``s[N ``-` `1``] ``+``=` `1` `    ``ans ``=` `""``    ``for` `i ``in` `range``(``len``(s)):``        ``ans ``+``=` `s[i]``        ` `    ``# Return the resultant String``    ``return` `ans` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``    ` `    ``# Given String S``    ``S ``=` `"?a?a"` `    ``# Function Call``    ``print``(changeString(S))` `# This code is contributed by gauravrajput1`

## C#

 `// C# program for the above approach``using` `System;`` ` `class` `GFG{` `// Function that replace all '?' with``// lowercase alphabets such that each``// adjacent character is different``static` `string` `changeString(``string` `S)``{``  ` `  ``// Store the given String``  ``char` `[]s = S.ToCharArray();``  ` `  ``int` `N = S.Length;` `  ``// If the first character is '?'``  ``if` `(s == ``'?'``)``  ``{``    ``s = ``'a'``;``    ``if` `(s == s)``    ``{``      ``s++;``    ``}``  ``}` `  ``// Traverse the String [1, N - 1]``  ``for``(``int` `i = 1; i < N - 1; i++)``  ``{``    ` `    ``// If the current``    ``// character is '?'``    ``if` `(s[i] == ``'?'``)``    ``{``      ` `      ``// Change the character``      ``s[i] = ``'a'``;` `      ``// Check equality with``      ``// the previous character``      ``if` `(s[i] == s[i - 1])``      ``{``        ``s[i]++;``      ``}` `      ``// Check equality with``      ``// the next character``      ``if` `(s[i] == s[i + 1])``      ``{``        ``s[i]++;``      ``}` `      ``// Check equality with``      ``// the previous character``      ``if` `(s[i] == s[i - 1])``      ``{``        ``s[i]++;``      ``}``    ``}``  ``}` `  ``// If the last character is '?'``  ``if` `(s[N - 1] == ``'?'``)``  ``{``    ` `    ``// Change character``    ``s[N - 1] = ``'a'``;` `    ``// Check with previous``    ``// character``    ``if` `(s[N - 1] == s[N - 2])``    ``{``      ``s[N - 1]++;``    ``}``  ``}` `  ``string` `ans = ``""``;``  ` `  ``for``(``int`  `i = 0; i < s.Length; i++)``  ``{``    ``ans += s[i];``  ``}``  ` `  ``// Return the resultant String``  ``return` `ans;``}` `// Driver Code``public` `static` `void` `Main()``{``  ` `  ``// Given String S``  ``string` `S = ``"?a?a"``;` `  ``// Function Call``  ``Console.WriteLine(changeString(S));``}``}` `// This code is contributed by sanjoy_62`

## Javascript

 ``

Output

`baba`

Time Complexity: O(N), where N is the length of the given string.
Auxiliary Space: O(N)

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