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

In this problem, we are given string str, and Q queries, each consisting of two integers. Our task is to create the program to solve Queries to find the last non-repeating character in the sub-string of a given string in C++.

Problem Description

In each query, we have two integers L and R. To solve the queries, we will take a substring starting from index L to index R. And find the last character which is non-repeating in the sub-string.

Let’s take an example to understand the problem,

Input: str = “Tutorialspoint” Q = 2

query = {{4, 8}, {2, 6}}

Output: -1 , -1

Explanation

subStr[4...8] = “rials”. The last non-repeating character is s. But all characters have frequency 1.

subStr[2...6] = “toria”. The last non-repeating character is a. But all characters have frequency 1.

Solution Approach

To solve the problem, we need to find the character with a single frequency of occurrence. For this, an easy and simple approach is to create a matrix to calculate the charFreq[][]. To solve the subarray query, we will check for the frequency of occurrence of all characters and return the last character with frequency 1.

Example

 Live Demo

#include<bits/stdc++.h>
using namespace std;
int charFreq[256][1000] = {0};
void initialiseCharFrequency(string str, int n) {
   charFreq[(int)str[0]][0] = 1;
   for (int i = 1; i < n; i++) {
      char ch = str[i];
      for (int j = 0; j < 256; j++) {
         char charToUpdate = (char)j;
         if (charToUpdate == ch)
            charFreq[j][i] = charFreq[j][i - 1] + 1;
         else
            charFreq[j][i] = charFreq[j][i - 1];
      }
   }
}
string returnCharFromString(char x) {
   string s(1, x);
   return s;
}
string lastUniqueChar(string str, int n, int start, int end) {
   for (int i = end; i >= start; i--) {
      char ch = str[i];
   if ((charFreq[(int)ch][end] - charFreq[(int)ch][start - 1]) ==1)
      return returnCharFromString(ch);
   }
   return "-1";
}
int main() {
   string str = "TutorialsPoint";
   int len = str.length();
   int Q = 3;
   int query[Q][2] = { { 2, 9 }, { 2, 3 }, { 0, 12 } };
   initialiseCharFrequency(str, len);
   for (int i = 0; i < Q; i++)
      cout<<"\nFor Query "<<(i+1)<<": The last non-repeating character in the sub-string of a given string is "<<lastUniqueChar(str, len,query[i][0], query[i][1]);
}

Output

For Query 1: The last non-repeating character in the sub-string of a given string is P
For Query 2: The last non-repeating character in the sub-string of a given string is o
For Query 3: The last non-repeating character in the sub-string of a given string is n