first non-repeating character.

This problem was asked by SAP Labs.

Given a string of characters, return the index of the first non-repeating character.

Example
function nonRepeatingFirstCharacter(str) {

}

const str = "abcdcaf";
console.log(nonRepeatingFirstCharacter(str)); // 1
Explanation: The non-duplicate characters in the above string are b, d and f. Since b occurs first, the index of b, which is 1 is returned.

Solution

function nonRepeatingFirstCharacter(str) {
  const map = new Map();
  for (const char of str) {
    map.set(char, (map.get(char) || 0) + 1);
  }
  for (let i = 0; i < str.length; i++) {
    const char = str[i];
    if (map.get(char) === 1) return i;
  }
  return -1;
}

const str = "abcdcaf";
console.log(nonRepeatingFirstCharacter(str)); // 1

Explanation:

• Use a hash table (object) to store count of each character in one pass
• Increment count if character is already in table, default to 1
• Then loop through string again checking if char count is 1
• Return index immediately if non-duplicate char found