排序算法

请写一个 min 函数，要求 min(numbers) 能返回数组 numbers 中的最小数字。

let min = (numbers) => {
  if(numbers.length > 2){
    return min(
      [numbers[0], min(numbers.slice(1))]
    )
  }else{
    return Math.min.apply(null, numbers)
  }
}

let sort = (numbers) => {
  if(numbers.length > 2){
    let index = minIndex(numbers)
    let min = numbers[index]
    numbers.splice(index, 1)
    return [min].concat(sort(numbers))
    }else{
    return numbers[0]<numbers[1] ? numbers :
           numbers.reverse()
  }
}

请写出一个 sort 函数，要求 sort(numbers) 能返回一个把 numbers 从小到大排列的数组（选择排序）

let minIndex = (numbers) =>
  let index = 0
  for(let i=1; i<numbers.length; i++){
    if(numbers[i] < numbers[index]){
      index = i
    }
  }
  return index
}
let sort = (numbers) => {
  for(let i=0; i<???; i++){
    let index = minIndex(numbers)
    // index 是当前最小数的下标
    // index 对应的数应该放到 i 处
    swap(numbers, index, i) // swap 还没实现
    // index、i 都是 index 的意思，建议 i 改名
  }
}
let swap = (array, i, j) => {
  let temp = array[i]
  array[i] = array[j]
  array[j] = temp
}
swap(numbers, 1, 2)

四种排序算法

选择排序的循环写法

let sort = (numbers) => {
  for(let i=0; i< numbers.length -1; i++){
    console.log(`----`) //这个log很精髓
    console.log(`i: ${i}`)
    let index = minIndex(numbers.slice(i))+ i
    console.log(`index: ${index}`)
    console.log(`min: ${numbers[index]}`)
    if(index!==i){
      swap(numbers, index, i)
      console.log(`swap ${index}: ${i}`)
      console.log(numbers)
    }
}
  return numbers
}

let swap = (array, i, j) => {
  let temp = array[i]
  array[i] = array[j]
  array[j] = temp
}
let minIndex = (numbers) => {
  let index = 0
  for(let i=1; i<numbers.length; i++){
    if(numbers[i] < numbers[index]){
      index = i
    }
  }
  return index
}

快速排序

let quickSort = arr => {
　　if (arr.length <= 1) { return arr; }
　　let pivotIndex = Math.floor(arr.length / 2);
　　let pivot = arr.splice(pivotIndex, 1)[0];
　　let left = [];
　　let right = [];
　　for (let i = 0; i < arr.length; i++){
　　　　if (arr[i] < pivot) { left.push(arr[i])
　　　　} else { right.push(arr[i]) }
　　}
　　return quickSort(left).concat(
             [pivot], quickSort(right) )
}

归并排序

let mergeSort = arr =>{
  let k = arr.length
  if(k===1){return arr}
  let left = arr.slice(0, Math.floor(k/2))
  let right = arr.slice(Math.floor(k/2))
  return merge(mergeSort(left), mergeSort(right))
}
let merge = (a, b) => {
  if(a.length === 0) return b
  if(b.length === 0) return a
  return a[0] > b[0] ?
     [b[0]].concat(merge(a, b.slice(1))) :
     [a[0]].concat(merge(a.slice(1), b))
}

计数排序(最快)

用了hashTable哈希表(key:value)，只编译数组一次(不过还要遍历一次hashTable)，这叫用空间换时间。

let countSort = arr =>{
  let hashTable = {}, max = 0, result = []
  for(let i=0; i<arr.length; i++){ // 遍历数组
    if(!(arr[i] in hashTable)){ 
      hashTable[arr[i]] = 1
    }else{
      hashTable[arr[i]] += 1
    }
    if(arr[i] > max) {max = arr[i]}
  }
  for(let j=0; j<=max; j++){ // 遍历哈希表
    if(j in hashTable){
      for(let i = 0; i<hashTable[j]; i++){
        result.push(j)
      }
    }
  }
  return result
}