Rearrange characters in a string such that no two adjacent are same

Given a string with repeated characters, the task is to rearrange characters in a string so that no two adjacent characters are same.
Note : It may be assumed that the string has only lowercase English alphabets.

Examples:  

Input: aaabc 
Output: abaca 

Input: aaabb
Output: ababa 

Input: aa 
Output: Not Possible

Input: aaaabc 
Output: Not Possible

Asked In : Amazon Interview

Approach (using priority_queue):
The idea is to put the highest frequency character first (a greedy approach). We use a priority queue (Or Binary Max Heap) and put all characters and ordered by their frequencies (highest frequency character at root). We one by one take the highest frequency character from the heap and add it to result. After we add, we decrease the frequency of the character and we temporarily move this character out of priority queue so that it is not picked next time.

We have to follow the step to solve this problem, they are: 

• Build a Priority_queue or max_heap, pq that stores characters and their frequencies. 
  • Priority_queue or max_heap is built on the bases of the frequency of character. 
• Create a temporary Key that will be used as the previously visited element (the previous element in the resultant string. Initialize it { char = ‘#’ , freq = ‘-1’ } 
• While pq is not empty. 
  • Pop an element and add it to the result. 
  • Decrease frequency of the popped element by ‘1’ 
  • Push the previous element back into the priority_queue if it’s frequency > ‘0’ 
  • Make the current element as the previous element for the next iteration. 
• If the length of the resultant string and original string is not equal, print “not possible”. Else print result.

Below is the implementation of above idea:

C++

// C++ program to rearrange characters in a string
// so that no two adjacent characters are same.
#include <bits/stdc++.h>
using namespace std;

const int MAX_CHAR = 26;

struct Key {
    int freq; // store frequency of character
    char ch;

    // function for priority_queue to store Key
    // according to freq
    bool operator<(const Key& k) const
    {
        return freq < k.freq;
    }
};

// Function to rearrange character of a string
// so that no char repeat twice
void rearrangeString(string str)
{
    int n = str.length();

    // Store frequencies of all characters in string
    int count[MAX_CHAR] = { 0 };
    for (int i = 0; i < n; i++)
        count[str[i] - 'a']++;

    // Insert all characters with their frequencies
    // into a priority_queue
    priority_queue<Key> pq;
    for (char c = 'a'; c <= 'z'; c++) {
          int val = c - 'a';
        if (count[val]) {
            pq.push(Key{ count[val], c });
        }
    }

    // 'str' that will store resultant value
    str = "";

    // work as the previous visited element
    // initial previous element be. ( '#' and
    // it's frequency '-1' )
    Key prev{ -1, '#' };

    // traverse queue
    while (!pq.empty()) {
        // pop top element from queue and add it
        // to string.
        Key k = pq.top();
        pq.pop();
        str = str + k.ch;

        // IF frequency of previous character is less
        // than zero that means it is useless, we
        // need not to push it
        if (prev.freq > 0)
            pq.push(prev);

        // make current character as the previous 'char'
        // decrease frequency by 'one'
        (k.freq)--;
        prev = k;
    }

    // If length of the resultant string and original
    // string is not same then string is not valid
    if (n != str.length())
        cout << " Not valid String " << endl;

    else // valid string
        cout << str << endl;
}

// Driver program to test above function
int main()
{
    string str = "bbbaa";
    rearrangeString(str);
    return 0;
}

Java

// Java program to rearrange characters in a string
// so that no two adjacent characters are same.
import java.io.*;
import java.util.*;

class KeyComparator implements Comparator<Key> {

    // Overriding compare()method of Comparator
    public int compare(Key k1, Key k2)
    {
        if (k1.freq < k2.freq)
            return 1;
        else if (k1.freq > k2.freq)
            return -1;
        return 0;
    }
}

class Key {
    int freq; // store frequency of character
    char ch;
    Key(int val, char c)
    {
        freq = val;
        ch = c;
    }
}

class GFG {
    static int MAX_CHAR = 26;

    // Function to rearrange character of a string
    // so that no char repeat twice
    static void rearrangeString(String str)
    {
        int n = str.length();

        // Store frequencies of all characters in string
        int[] count = new int[MAX_CHAR];

        for (int i = 0; i < n; i++)
            count[str.charAt(i) - 'a']++;

        // Insert all characters with their frequencies
        // into a priority_queue
        PriorityQueue<Key> pq
            = new PriorityQueue<>(new KeyComparator());
        for (char c = 'a'; c <= 'z'; c++) {
            int val = c - 'a';
            if (count[val] > 0)
                pq.add(new Key(count[val], c));
        }

        // 'str' that will store resultant value
        str = "";

        // work as the previous visited element
        // initial previous element be. ( '#' and
        // it's frequency '-1' )
        Key prev = new Key(-1, '#');

        // traverse queue
        while (pq.size() != 0) {

            // pop top element from queue and add it
            // to string.
            Key k = pq.peek();
            pq.poll();
            str = str + k.ch;

            // If frequency of previous character is less
            // than zero that means it is useless, we
            // need not to push it
            if (prev.freq > 0)
                pq.add(prev);

            // make current character as the previous 'char'
            // decrease frequency by 'one'
            (k.freq)--;
            prev = k;
        }

        // If length of the resultant string and original
        // string is not same then string is not valid
        if (n != str.length())
            System.out.println(" Not valid String ");
        else
            System.out.println(str);
    }

    // Driver program to test above function
    public static void main(String args[])
    {
        String str = "bbbaa";
        rearrangeString(str);
    }
}

// This code is contributed by rachana soma

Python3

# Python program to rearrange characters in a string
# so that no two adjacent characters are same.
from heapq import heappush, heappop
from collections import Counter

# A key class for readability
class Key:
    def __init__(self, character: str, freq: int) -> None:
        self.character = character
        self.freq = freq

    def __lt__(self, other: "Key") -> bool:
        return self.freq > other.freq


# Function to rearrange character of a string
# so that no char repeat twice
def rearrangeString(str: str):
    n = len(str)
    # creating a frequency hashmap
    count = dict()
    for i in str:
        count[ord(i)] = count.get(ord(i), 0) + 1

    pq = []
    for c in range(97, 123):
        if count.get(c, 0):
            heappush(pq, Key(chr(c), count))

    # null character for default previous checking
    prev = Key('#', -1)
    str = ""

    while pq:
        key = heappop(pq)
        str += key.character

        # Since one character is already added
        key.freq -= 1

        # We avoid inserting if the frequency drops to 0
        if prev.freq > 0:
            heappush(pq, prev)

        prev = key

    if len(str) != n:
        print("Not a Valid str")
    else:
        print(str)


# Driver Code
if __name__ == "__main__":
    string = "bbbaa"
    rearrangeString(string)

    # This code is contributed by kraanzu.

Output

babab

Time complexity : O(n log(n))

Here n is length of the string

Auxiliary Space: O(n)

Extra space is used to store the resultant string, other spaces used are constant.

Another approach: Another approach is to fill all the even positions of the result string first, with the highest frequency character. If there are still some even positions remaining, fill them first. Once even positions are done, then fill the odd positions. This way, we can ensure that no two adjacent characters are the same. 

C++14

#include <bits/stdc++.h>
using namespace std;

char getMaxCountChar(const vector<int>& count)
{
    int max = 0;
    char ch;
    for (int i = 0; i < 26; i++) {
        if (count[i] > max) {
            max = count[i];
            ch = 'a' + i;
        }
    }

    return ch;
}

string rearrangeString(string S)
{

    int n = S.size();
    if (!n)
        return "";

    vector<int> count(26, 0);
    for (auto ch : S)
        count[ch - 'a']++;

    char ch_max = getMaxCountChar(count);
    int maxCount = count[ch_max - 'a'];

    // check if the result is possible or not
    if (maxCount > (n + 1) / 2)
        return "";

    string res(n, ' ');

    int ind = 0;
    // filling the most frequently occurring char in the even
    // indices
    while (maxCount) {
        res[ind] = ch_max;
        ind = ind + 2;
        maxCount--;
    }
    count[ch_max - 'a'] = 0;

    // now filling the other Chars, first filling the even
    // positions and then the odd positions
    for (int i = 0; i < 26; i++) {
        while (count[i] > 0) {
            ind = (ind >= n) ? 1 : ind;
            res[ind] = 'a' + i;
            ind += 2;
            count[i]--;
        }
    }
    return res;
}

// Driver program to test above function
int main()
{
    string str = "bbbaa";
    string res = rearrangeString(str);
    if (res == "")
        cout << "Not valid string" << endl;
    else
        cout << res << endl;
    return 0;
}

Java

/*package whatever //do not write package name here */

import java.io.*;

class GFG {
  static char getMaxCountChar(int[] count)
  {
    int max = 0;
    char ch = 0;
    for (int i = 0; i < 26; i++) {
      if (count[i] > max) {
        max = count[i];
        ch = (char)((int)'a' + i);
      }
    }
    return ch;
  }

  static String rearrangeString(String S)
  {

    int n = S.length();
    if (n==0)
      return "";

    int[]count = new int[26];
    for(int i=0;i<26;i++){
      count[i] = 0;
    }
    for (char ch : S.toCharArray()){
      count[(int)ch - (int)'a']++;
    }


    char ch_max = getMaxCountChar(count);
    int maxCount = count[(int)ch_max - (int)'a'];

    // check if the result is possible or not
    if (maxCount > (n + 1) / 2)
      return "";

    String res = "";
    for(int i=0;i<n;i++){
      res += ' ';
    }

    int ind = 0;
    // filling the most frequently occurring char in the even
    // indices
    while (maxCount > 0) {
      res = res.substring(0,ind) + ch_max + res.substring(ind+1);
      ind = ind + 2;
      maxCount--;
    }
    count[(int)ch_max - (int)'a'] = 0;

    // now filling the other Chars, first filling the even
    // positions and then the odd positions
    for (int i = 0; i < 26; i++) {
      while (count[i] > 0) {
        ind = (ind >= n) ? 1 : ind;
        res = res.substring(0,ind) + (char)((int)'a' + i) + res.substring(ind+1);
        ind += 2;
        count[i]--;
      }
    }
    return res;
  }

  // Driver Code
  public static void main(String args[])
  {
    String str = "bbbaa";
    String res = rearrangeString(str);
    if (res == "")
      System.out.println("Not valid string");
    else
      System.out.println(res);
  }
}

// This code is contributed by shinjanpatra

Python3

# Python program for rearranging characters in a string such
# that no two adjacent are same

# Function to find the char with maximum frequency in the given
# string
def getMaxCountChar(count):
  maxCount = 0
  for i in range(26):
    if count[i] > maxCount:
        maxCount = count[i]
        maxChar = chr(i + ord('a'))

  return maxCount, maxChar

# Main function for rearranging the characters
def rearrangeString(S):
  n = len(S)

  # if length of string is None return False
  if not n:
      return False

  # create a hashmap for the alphabets
  count = [0] * 26
  for char in S:
      count[ord(char) - ord('a')] += 1


  maxCount, maxChar = getMaxCountChar(count)

  # if the char with maximum frequency is more than the half of the
  # total length of the string than return False
  if maxCount > (n + 1) // 2:
      return False

  # create a list for storing the result
  res = [None] * n

  ind = 0

  # place all occurrences of the char with maximum frequency in
  # even positions
  while maxCount:
      res[ind] = maxChar
      ind += 2
      maxCount -= 1

  # replace the count of the char with maximum frequency to zero
  # as all the maxChar are already placed in the result
  count[ord(maxChar) - ord('a')] = 0

  # place all other char in the result starting from remaining even
  # positions and then place in the odd positions
  for i in range(26):
      while count[i] > 0:
          if ind >= n:
              ind = 1
          res[ind] = chr(i + ord('a') )
          ind += 2
          count[i] -= 1


  # convert the result list to string and return
  return ''.join(res)

# Driver Code
str = 'bbbaa'
res = rearrangeString(str)
if res:
  print(res)
else:
  print('Not valid string')

# This code is contributed by Manish Thapa

Javascript

<script>

// JavaScript program for rearranging characters in a string such
// that no two adjacent are same

// Function to find the char with maximum frequency in the given
// string
function getMaxCountChar(count){
let maxCount = 0
let maxChar
for(let i = 0; i < 26; i++){
    if(count[i] > maxCount){
        maxCount = count[i]
        maxChar = String.fromCharCode(i + ('a').charCodeAt(0))
  }
}

return [maxCount, maxChar]
}

// Main function for rearranging the characters
function rearrangeString(S){
let n = S.length

// if length of string is None return false
if(!n)
    return false

// create a hashmap for the alphabets
let count = new Array(26).fill(0)
for(let char of S)
    count[char.charCodeAt(0) - ('a').charCodeAt(0)] += 1

let [maxCount, maxChar] = getMaxCountChar(count)

// if the char with maximum frequency is more than the half of the
// total length of the string than return false
if(maxCount > Math.floor((n + 1) / 2))
    return false

// create a list for storing the result
let res = new Array(n)

let ind = 0

// place all occurrences of the char with maximum frequency in
// even positions
while(maxCount){
    res[ind] = maxChar
    ind += 2
    maxCount -= 1
}

// replace the count of the char with maximum frequency to zero
// as all the maxChar are already placed in the result
count[maxChar.charCodeAt(0) - 'a'.charCodeAt(0)] = 0

// place all other char in the result starting from remaining even
// positions and then place in the odd positions
for(let i = 0; i < 26; i++)
{
    while(count[i] > 0)
    {
        if(ind >= n)
            ind = 1
        res[ind] = String.fromCharCode(i + ('a').charCodeAt(0))
        ind += 2
        count[i] -= 1
  }
}

// convert the result list to string and return
return res.join('')
}

// Driver Code
let str = 'bbbaa'
let res = rearrangeString(str)
if(res)
  document.write(res,"</br>")
else
  document.write('Not valid string',"</br>")

// This code is contributed by shinjanpatra

</script>

Output

babab

Time complexity : O(n)
Auxiliary Space : O(n+26) where 26 is the size of the vocabulary. 

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