二分查找

递归实现

var arr = [1, 2, 4, 5, 6, 7, 39]

function binSearch(start, end, arr, target) {
  let mid = Math.floor((end - start) / 2 )+ start
  if (start > end ) return - 1
  console.log(start, end, mid)
  if (arr[mid] === target) {
    return mid
  } else if (arr[mid] < target) {
    return binSearch(mid + 1, end, arr, target)
  } else {
    return binSearch(start, mid - 1, arr, target) 
  }
  return -1
}

非递归实现

var arr = [1, 2, 4, 5, 6, 7, 39]

function binSearch(arr, target) {
  let start = 0
  let end = arr.length
  while(start <= end) {
    let mid = Math.floor((end - start) / 2)+ start
    if (arr[mid] === target) {
      return mid
    } else if (arr[mid] < target) {
      start = mid + 1
    } else {
      end = mid - 1
    }
  }
}

binSearch(arr, 6)

为什么这么简单的题都答不出来。。