如何验证数组中的字母是否重复？

function hasDuplicateLetters(arr) {
  const uniqueLetters = new Set(arr);
  return uniqueLetters.size !== arr.length;
}

// 示例
const array1 = ['a', 'b', 'c'];
const array2 = ['a', 'b', 'a'];

console.log(hasDuplicateLetters(array1)); // 输出: false
console.log(hasDuplicateLetters(array2)); // 输出: true

function hasDuplicateLetters(arr) {
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      if (arr[i] === arr[j]) {
        return true;
      }
    }
  }
  return false;
}

// 示例
const array1 = ['a', 'b', 'c'];
const array2 = ['a', 'b', 'a'];

console.log(hasDuplicateLetters(array1)); // 输出: false
console.log(hasDuplicateLetters(array2)); // 输出: true

function hasDuplicateLetters(arr) {
  for (let i = 0; i < arr.length; i++) {
    if (arr.indexOf(arr[i]) !== arr.lastIndexOf(arr[i])) {
      return true;
    }
  }
  return false;
}

// 示例
const array1 = ['a', 'b', 'c'];
const array2 = ['a', 'b', 'a'];

console.log(hasDuplicateLetters(array1)); // 输出: false
console.log(hasDuplicateLetters(array2)); // 输出: true

function hasDuplicateLetters(arr) {
  const letterCount = {};
  for (let i = 0; i < arr.length; i++) {
    if (letterCount[arr[i]]) {
      return true;
    } else {
      letterCount[arr[i]] = 1;
    }
  }
  return false;
}

// 示例
const array1 = ['a', 'b', 'c'];
const array2 = ['a', 'b', 'a'];

console.log(hasDuplicateLetters(array1)); // 输出: false
console.log(hasDuplicateLetters(array2)); // 输出: true