一、写在前面
查找算法主要就两种算法，一个是顺序查找，另一个是二分查找，顺序查找的时间复杂度为O(n),二分查找的时间复杂度为O(logn)。
二、顺序查找

let arr = [5, 4, 3, 2, 6, 8, 9, 1]

  const Sequential = (array, value) => {
    let length = array.length
    for (let i = 0; i < length; i++) {
      if (value === array[i]) {
        return i
      }
    }
    return -1
  }

  console.log(Sequential(arr, 0))

三、二分查找
可以使用二分查找的条件是:数组必须是顺序排列的

let arr = [1, 2, 3, 4, 5, 6, 7, 8]

const binarySearch = (array, num) => {
  let length = array.length
  let left = 0
  let right = length - 1
  while (left <= right) {
    let mid = Math.floor((right + left) / 2)
    if (array[mid] < num) {
      left = mid + 1
    }else if(arr[mid] > num) {
      right = mid - 1
    }else {
      return mid
    }
  }
  return -1
}

let index = binarySearch(arr, 0)
console.log(index)