二分法查找

function binarySearch (arr, target) {
   var l =  0;
   var r = arr.length - 1;
   while (l <= r){
       var middle = l + (r - l)/2;
       if (arr[middle] == target) {
           return middle;
       }
       if (arr[middle] < target) {
           l = middle + 1;
       } else {
           r = middle - 1;
       }
   }
   return -1;
}