var arr = [];
    // 通过for 生成排好序的测试数组  for (var i = 10; i < 50; i++){
        arr.push(i * 2);
    }
    // 通过for 测试方法  for (var i = 0; i < 100; i++){
        console.log(seek(arr, i), i)
    }
    /**  * @param arr 要查找的数组  * @param num 要查找的元素  * @returns {*} 元素对应的下标，若不存在，返回-1  */  function seek(arr, num) {
        return check(0, arr.length)
        function check(start, end) {
            var diff = parseInt((end - start) / 2);
            var middle = start + diff;
  // console.log(arr[middle], middle, start, end, diff)  if (diff == 0){
                if (arr[middle] == num){
                    return middle;
                } else {
                    return -1;
                }
            }
            if (arr[middle] == num) {
                return middle;
            } else if(arr[middle] < num) {
                return check(middle, end);
            } else if(arr[middle] > num) {
                return check(start, middle);
            }
        }
    }