/**
 * 二分搜尋首先要保證陣列有序
 * @param arr
 * @param val
 */
function binarySearch(arr, val) {
  let low = 0;
  let high = arr.length - 1;

  // 需要控制好迴圈變數
  // 在 [low, high] 之間尋找
  while (low <= high) {
    let mid = Math.floor(low + high);
    if (arr[mid] === val) {
      return mid;
    } else if (arr[mid] >
 
 val) {
      high = mid - 1 ;
    } else {
      low = mid + 1;
    }
  }
}

const arr = [11, 12, 13, 14, 15, 16];
console.log('search value of 15: ', binarySearch(arr, 15));