Number of Larger Elements on right side in a string

Given a string, find count of number of larger alphabets for every character of the string.

Examples:

Input  : str = "abcd"
Output : 3 2 1 0
There are 3 greater characters on right of 'a',
2 greater characters on right of 'b', 1 greater
character on right of 'c' and 0 greater characters
on right of 'd'.
      
Input :  geeks
Output : 2 2 2 1 0

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

A naive approach is to use two for loops. First will keep track of each alphabet in string and second loop will be used to find no of larger alphabet according to ASCII values.

C++

// CPP program to find counts of right greater
// characters for every character.
#include <bits/stdc++.h>
using namespace std;

void printGreaterCount(string str)
{
    int len = str.length(), right[len] = { 0 };
    for (int i = 0; i < len; i++) 
        for (int j = i + 1; j < len; j++) 
            if (str[i] < str[j])
                right[i]++;

    for (int i = 0; i < len; i++)
        cout << right[i] << " ";
}

// Driver code
int main()
{
    string str = "abcd";
    printGreaterCount(str);
    return 0;
}

PHP

<?php
// PHP program to find counts 
// of right greater characters
// for every character.

function printGreaterCount($str)
{
    $len = strlen($str); 
    $right = array_fill(0, $len, 0);
    for ($i = 0; $i < $len; $i++) 
    {
        for ($j = $i + 1; $j < $len; $j++) 
            if ($str[$i] < $str[$j])
                $right[$i]++;
    }

    for ($i = 0; $i < $len; $i++)
        echo $right[$i] . " ";
}

// Driver code
$str = 'abcd';
printGreaterCount($str);

// This code is contributed 
// by Abhinav96
?>

Output:

3 2 1 0

Time Complexity : O(N * N)

An efficient approach is to traverse the string from right and keep track of counts of characters from right side. For every character that we traverse from right, we increment its count in count array and add counts of all greater characters to answer for this character.

C++

// C++ program to count greater characters on right
// side of every character.
#include <bits/stdc++.h>
using namespace std;
#define MAX_CHAR 26

void printGreaterCount(string str)
{
    int len = str.length();
 
    // Arrays to store result and character counts.
    int ans[len] = { 0 }, count[MAX_CHAR] = { 0 };

    // start from right side of string
    for (int i = len - 1; i >= 0; i--) 
    {
        count[str[i] - 'a']++;
        for (int j = str[i] - 'a' + 1; j < MAX_CHAR; j++)
            ans[i] += count[j];
    }

    for (int i = 0; i < len; i++)
        cout << ans[i] << " ";
}

// Driver code
int main()
{
    string str = "abcd";
    printGreaterCount(str);
    return 0;
}

Output:

3 2 1 0

Time Complexity : O(N)

My Personal Notes

Ronin

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