# Queries to find the last non-repeating character in the sub-string of a given string

• Difficulty Level : Hard
• Last Updated : 06 Oct, 2021

Given a string str, the task is to answer Q queries where every query consists of two integers L and R and we have to find the last non-repeating character in the sub-string str[L…R]. If there is no non-repeating character then print -1.
Examples:

Input: str = “GeeksForGeeks”, q[] = {{2, 9}, {2, 3}, {0, 12}}
Output:

Sub-string for the queries are “eksForGe”, “ek” and “GeeksForGeeks” and their last non-repeating characters are ‘G’, ‘k’ and ‘r’ respectively.
‘G’ is the first character from the end in given range which has frequency 1.
Input: str = “xxyyxx”, q[] = {{2, 3}, {3, 4}}
Output:
-1

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Approach: Create a frequency array freq[][] where freq[i][j] stores the frequency of the character in the sub-string str[0…j] whose ASCII value is i. Now, frequency of any character in the sub-string str[i…j] whose ASCII value is x can be calculated as freq[x][j] = freq[x][i – 1]
Now for every query, start traversing the string in the given range i.e. str[L…R] and for every character, if its frequency is 1 then this is the last non-repeating character in the required sub-string. If all the characters have frequency greater than 1 then print -1.
Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include``using` `namespace` `std;` `// Maximum distinct characters possible``int` `MAX = 256;` `// To store the frequency of the characters``int` `freq = {0};` `// Function to pre-calculate the frequency array``void` `preCalculate(string str, ``int` `n)``{` `    ``// Only the first character has``    ``// frequency 1 till index 0``    ``freq[(``int``)str] = 1;` `    ``// Starting from the second``    ``// character of the string``    ``for` `(``int` `i = 1; i < n; i++)``    ``{``        ``char` `ch = str[i];` `        ``// For every possible character``        ``for` `(``int` `j = 0; j < MAX; j++)``        ``{` `            ``// Current character under consideration``            ``char` `charToUpdate = (``char``)j;` `            ``// If it is equal to the character``            ``// at the current index``            ``if` `(charToUpdate == ch)``                ``freq[j][i] = freq[j][i - 1] + 1;``            ``else``                ``freq[j][i] = freq[j][i - 1];``        ``}``    ``}``}` `// Function to return the frequency of the``// given character in the sub-string str[l...r]``int` `getFrequency(``char` `ch, ``int` `l, ``int` `r)``{` `    ``if` `(l == 0)``        ``return` `freq[(``int``)ch][r];``    ``else``        ``return` `(freq[(``int``)ch][r] - freq[(``int``)ch][l - 1]);``}`  `string getString(``char` `x)``{``    ` `    ``// string class has a constructor``    ``// that allows us to specify size of``    ``// string as first parameter and character``    ``// to be filled in given size as second``    ``// parameter.``    ``string s(1, x);` `    ``return` `s;``}` `// Function to return the last non-repeating character``string lastNonRepeating(string str, ``int` `n, ``int` `l, ``int` `r)``{` `    ``// Starting from the last character``    ``for` `(``int` `i = r; i >= l; i--)``    ``{` `        ``// Current character``        ``char` `ch = str[i];` `        ``// If frequency of the current character is 1``        ``// then return the character``        ``if` `(getFrequency(ch, l, r) == 1)``            ``return` `getString(ch);``    ``}` `    ``// All the characters of the``    ``// sub-string are repeating``    ``return` `"-1"``;``}` `// Driver code``int` `main()``{``    ``string str = ``"GeeksForGeeks"``;``    ``int` `n = str.length();` `    ``int` `queries = { { 2, 9 }, { 2, 3 }, { 0, 12 } };``    ``int` `q =3;` `    ``// Pre-calculate the frequency array``    ` `    ``preCalculate(str, n);` `    ``for` `(``int` `i = 0; i < q; i++)``    ``{``        ``cout << (lastNonRepeating(str, n,``                            ``queries[i],``                            ``queries[i]))<

## Java

 `// Java implementation of the approach``public` `class` `GFG {` `    ``// Maximum distinct characters possible``    ``static` `final` `int` `MAX = ``256``;` `    ``// To store the frequency of the characters``    ``static` `int` `freq[][];` `    ``// Function to pre-calculate the frequency array``    ``static` `void` `preCalculate(String str, ``int` `n)``    ``{` `        ``// Only the first character has``        ``// frequency 1 till index 0``        ``freq[(``int``)str.charAt(``0``)][``0``] = ``1``;` `        ``// Starting from the second``        ``// character of the string``        ``for` `(``int` `i = ``1``; i < n; i++) {``            ``char` `ch = str.charAt(i);` `            ``// For every possible character``            ``for` `(``int` `j = ``0``; j < MAX; j++) {` `                ``// Current character under consideration``                ``char` `charToUpdate = (``char``)j;` `                ``// If it is equal to the character``                ``// at the current index``                ``if` `(charToUpdate == ch)``                    ``freq[j][i] = freq[j][i - ``1``] + ``1``;``                ``else``                    ``freq[j][i] = freq[j][i - ``1``];``            ``}``        ``}``    ``}` `    ``// Function to return the frequency of the``    ``// given character in the sub-string str[l...r]``    ``static` `int` `getFrequency(``char` `ch, ``int` `l, ``int` `r)``    ``{` `        ``if` `(l == ``0``)``            ``return` `freq[(``int``)ch][r];``        ``else``            ``return` `(freq[(``int``)ch][r] - freq[(``int``)ch][l - ``1``]);``    ``}` `    ``// Function to return the last non-repeating character``    ``static` `String lastNonRepeating(String str, ``int` `n, ``int` `l, ``int` `r)``    ``{` `        ``// Starting from the last character``        ``for` `(``int` `i = r; i >= l; i--) {` `            ``// Current character``            ``char` `ch = str.charAt(i);` `            ``// If frequency of the current character is 1``            ``// then return the character``            ``if` `(getFrequency(ch, l, r) == ``1``)``                ``return` `(``""` `+ ch);``        ``}` `        ``// All the characters of the``        ``// sub-string are repeating``        ``return` `"-1"``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String str = ``"GeeksForGeeks"``;``        ``int` `n = str.length();` `        ``int` `queries[][] = { { ``2``, ``9` `}, { ``2``, ``3` `}, { ``0``, ``12` `} };``        ``int` `q = queries.length;` `        ``// Pre-calculate the frequency array``        ``freq = ``new` `int``[MAX][n];``        ``preCalculate(str, n);` `        ``for` `(``int` `i = ``0``; i < q; i++) {``            ``System.out.println(lastNonRepeating(str, n,``                                                ``queries[i][``0``],``                                                ``queries[i][``1``]));``        ``}``    ``}``}`

## Python3

 `# Python3 implementation of the approach` `# Maximum distinct characters possible``MAX` `=` `256` `# To store the frequency of the characters``freq ``=` `[[``0` `for` `i ``in` `range``(``256``)]``           ``for` `j ``in` `range``(``1000``)]` `# Function to pre-calculate``# the frequency array``def` `preCalculate(string, n):` `    ``# Only the first character has``    ``# frequency 1 till index 0``    ``freq[``ord``(string[``0``])][``0``] ``=` `1` `    ``# Starting from the second``    ``# character of the string``    ``for` `i ``in` `range``(``1``, n):``        ``ch ``=` `string[i]` `        ``# For every possible character``        ``for` `j ``in` `range``(``MAX``):` `            ``# Current character under consideration``            ``charToUpdate ``=` `chr``(j)` `            ``# If it is equal to the character``            ``# at the current index``            ``if` `charToUpdate ``=``=` `ch:``                ``freq[j][i] ``=` `freq[j][i ``-` `1``] ``+` `1``            ``else``:``                ``freq[j][i] ``=` `freq[j][i ``-` `1``]` `# Function to return the frequency of the``# given character in the sub-string str[l...r]``def` `getFrequency(ch, l, r):``    ``if` `l ``=``=` `0``:``        ``return` `freq[``ord``(ch)][r]``    ``else``:``        ``return` `(freq[``ord``(ch)][r] ``-``                ``freq[``ord``(ch)][l ``-` `1``])` `# Function to return the``# last non-repeating character``def` `lastNonRepeating(string, n, l, r):` `    ``# Starting from the last character``    ``for` `i ``in` `range``(r, l ``-` `1``, ``-``1``):` `        ``# Current character``        ``ch ``=` `string[i]` `        ``# If frequency of the current character is 1``        ``# then return the character``        ``if` `getFrequency(ch, l, r) ``=``=` `1``:``            ``return` `ch` `    ``# All the characters of the``    ``# sub-string are repeating``    ``return` `"-1"` `# Driver Code``if` `__name__ ``=``=` `"__main__"``:``    ``string ``=` `"GeeksForGeeks"``    ``n ``=` `len``(string)` `    ``queries ``=` `[(``2``, ``9``), (``2``, ``3``), (``0``, ``12``)]``    ``q ``=` `len``(queries)` `    ``# Pre-calculate the frequency array``    ``preCalculate(string, n)` `    ``for` `i ``in` `range``(q):``        ``print``(lastNonRepeating(string, n,``                               ``queries[i][``0``],``                               ``queries[i][``1``]))` `# This code is contributed by``# sanjeev2552`

## C#

 `// C# implementation of the approach``using` `System;` `class` `GFG``{` `    ``// Maximum distinct characters possible``    ``static` `int` `MAX = 256;` `    ``// To store the frequency of the characters``    ``static` `int` `[,]freq;` `    ``// Function to pre-calculate the frequency array``    ``static` `void` `preCalculate(``string` `str, ``int` `n)``    ``{` `        ``// Only the first character has``        ``// frequency 1 till index 0``        ``freq[(``int``)str,0] = 1;` `        ``// Starting from the second``        ``// character of the string``        ``for` `(``int` `i = 1; i < n; i++)``        ``{``            ``char` `ch = str[i];` `            ``// For every possible character``            ``for` `(``int` `j = 0; j < MAX; j++)``            ``{` `                ``// Current character under consideration``                ``char` `charToUpdate = (``char``)j;` `                ``// If it is equal to the character``                ``// at the current index``                ``if` `(charToUpdate == ch)``                    ``freq[j, i] = freq[j, i - 1] + 1;``                ``else``                    ``freq[j, i] = freq[j, i - 1];``            ``}``        ``}``    ``}` `    ``// Function to return the frequency of the``    ``// given character in the sub-string str[l...r]``    ``static` `int` `getFrequency(``char` `ch, ``int` `l, ``int` `r)``    ``{` `        ``if` `(l == 0)``            ``return` `freq[(``int``)ch, r];``        ``else``            ``return` `(freq[(``int``)ch, r] - freq[(``int``)ch, l - 1]);``    ``}` `    ``// Function to return the last non-repeating character``    ``static` `String lastNonRepeating(``string` `str, ``int` `n, ``int` `l, ``int` `r)``    ``{` `        ``// Starting from the last character``        ``for` `(``int` `i = r; i >= l; i--)``        ``{` `            ``// Current character``            ``char` `ch = str[i];` `            ``// If frequency of the current character is 1``            ``// then return the character``            ``if` `(getFrequency(ch, l, r) == 1)``                ``return` `(``""` `+ ch);``        ``}` `        ``// All the characters of the``        ``// sub-string are repeating``        ``return` `"-1"``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``string` `str = ``"GeeksForGeeks"``;``        ``int` `n = str.Length;` `        ``int` `[,]queries = { { 2, 9 }, { 2, 3 }, { 0, 12 } };``        ``int` `q = queries.Length;` `        ``// Pre-calculate the frequency array``        ``freq = ``new` `int``[MAX,n];``        ``preCalculate(str, n);` `        ``for` `(``int` `i = 0; i < q; i++)``        ``{``            ``Console.WriteLine(lastNonRepeating(str, n,``                                                ``queries[i,0],``                                                ``queries[i,1]));``        ``}``    ``}``}` `// This code is contributed by AnkitRai01`

## PHP

 `= ``\$l``; ``\$i``--)``    ``{` `        ``// Current character``        ``\$ch` `= ``\$str``[``\$i``];` `        ``// If frequency of the current character is 1``        ``// then return the character``        ``if` `(getFrequency(``\$ch``, ``\$l``, ``\$r``) == 1)``             ``return` `\$ch``;``    ``}` `    ``// All the characters of the``    ``// sub-string are repeating``    ``return` `"-1"``;``}` `// Driver code` `\$str` `= ``"GeeksForGeeks"``;``\$n` `= ``strlen``(``\$str``);` `\$queries` `= ``array``( ``array``( 2, 9 ), ``array``( 2, 3 ), ``array``( 0, 12 ) );` `\$q` `=3;` `// Pre-calculate the frequency array` `preCalculate(``\$str``, ``\$n``);` `for` `(``\$i` `= 0; ``\$i` `< ``\$q``; ``\$i``++)``{``    ``echo` `(lastNonRepeating(``\$str``, ``\$n``,``                        ``\$queries``[``\$i``],``                        ``\$queries``[``\$i``])), ``"\n"``;``}`  `// This code is contributed by ihritik` `?>`

## Javascript

 ``
Output:
```G
k
r```

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