Algorithm

Traverse the string from left to right.
Keep track of the leftmost index of each character as you encounter them.
If a character repeats, compare its leftmost index with the current result (which is initially set to -1).
Update the result to the leftmost index if it is smaller than the current result, ensuring that the leftmost repeating character is chosen.

Code

#include 
using namespace std;

// define alphabet size
#define NO_OF_CHARS 256

// return index of the left-most repeating character, else -1
int firstRepeating(string& str) {
  // create first-index array
  int firstIndex[NO_OF_CHARS];
  // initialize with -1
  fill(firstIndex, firstIndex + NO_OF_CHARS, -1);
  // initialize result as +infinity
  int res = INT_MAX;
  // scan characters
  for (int i = 0; i < (int)str.length(); i++) {
    // convert char to unsigned index
    unsigned char ch = static_cast(str[i]);
    // set first index if unseen
    if (firstIndex[ch] == -1)
      firstIndex[ch] = i;
    // otherwise m