输入 目标值， 数组
    关键在于用两个指针，一前一后，控制数组变化的范围

    1. function binarySearch(target, arr) {
    2.     let start = 0
    3.     let end = arr.length - 1
    5.     while (start <= end) {
    6.         let mid = Math.floor(start + (end-start / 2))
    7.         if (target === arr[mid]) {
    8.             return mid
    9.         } else if (target < arr[mid]) {
    10.             end = mid - 1
    11.         } else {
    12.             start = mid + 1
    13.         }
    14.     }
    15.     return -1
    16. }
    18. const arr = [1,2,3,4,5,6]
    19. let result = binarySearch(2, arr)
    20. console.log(result)