二分查詢

條件：有序陣列
思路：

1. 每次都和陣列中間位置數進行比較
2. 等於則返回
3. 若比其小，則 high = mid -1
4. 比其大，則 low = mid + 1
5. 迴圈條件為 low <= high

程式碼實現

		//二分查詢，找到返回元素索引，否則返回-1
        function binary_search(arr, item) {
            let low = 0;
            let high = arr.length - 1;

            while( low <= high){
                let mid = parseInt((low + high)/2);
                if( item === arr[mid]){
                    return mid;
                }else if( item < arr[mid]){ 
                    high = mid - 1;
                }else if( item > arr[mid]){
                    low = mid + 1;
                }
            }

            return -1;
        }