First non-repeating character

5 kyu

題目描述

Description

Write a function named first_non_repeating_letter† that takes a string input, and returns the first character that is not repeated anywhere in the string.

For example, if given the input 'stress', the function should return 't', since the letter t only occurs once in the string, and occurs first in the string.

As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter. For example, the input 'sTreSS' should return 'T'.

If a string contains all repeating characters, it should return an empty string ("");

† Note: the function is called firstNonRepeatingLetter for historical reasons, but your function should handle any Unicode character.

解答

Solution

function firstNonRepeatingLetter(s) {
  const counter = {};
  const lowerStr = s.toLowerCase();

  for (let i = 0; i < lowerStr.length; i++) {
    if (counter[lowerStr[i]]) {
      counter[lowerStr[i]]++;
    } else {
      counter[lowerStr[i]] = 1;
    }
  }

  for (let i = 0; i < s.length; i++) {
    if (counter[lowerStr[i]] === 1) {
      return s[i];
    }
  }

  return "";
}

解題思路

使用 Counter 的作法簡化邏輯，因為題目有As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter. For example, the input 'sTreSS' should return 'T'.的關係，所以在使用 counter 統計的會是 lowerStr，但回傳的要是原本的 s 之中最早出現的那個 s[i]

心得

練習 Counter 的題目，也配養看到題目想出初步解法的能力。