检测字符串排序，错误返回相应的数字

function findMissingNumber(sequence){
  //your code here
  if(sequence.length===0) {
    return 0;
  }
  let numbersArr = sequence.split(" ");
  
  if(numbersArr[0]==1) {
    for (var i=1;i<numbersArr.length;i++) {
      if (numbersArr[i] === "a") {
        return i;
      }
      if(numbersArr[i] != (i+1)) {
        return (i+1);
      }
    }
    return 0
  }else {
    return 1
  }
}

题目