点名

某班级 n 位同学的学号为 0 ~ n-1。点名结果记录于升序数组 records。假定仅有一位同学缺席，请返回他的学号。

  • 输入: records = [0,1,2,3,5]
    输出: 4

  • 输入: records = [0, 1, 2, 3, 4, 5, 6, 8]
    输出: 7

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var takeAttendance = function(records) {
  let left = 0
  let right = records.length - 1
  let mid = left + ((right - left) >> 1)
  while(left <= right){
    if(mid === records[mid]){
      left = mid + 1
    } else {
      right = mid - 1
    }
    mid = left + ((right - left) >> 1)
  }
  return left 
};