有序查找——折半查找

　　

public static String binarySearch(int[] a, int num) {
        if (a.length == 0)
            return "Eroor";
        int start = 0;
        int end = a.length - 1;
        int mid = (start + end) / 2;
        while (start <= end) {
            if (num == a[mid]) {
                return "所查数在数组的下标为" + mid;
            } else if (num > a[mid]) {
                start = mid + 1;
            } else if (num < a[mid]) {
                end = mid - 1;
            }
            mid = (start + end) / 2;
        }
        return "数组中无该数值";
    }

递归实现：

public static String binarySearch(int[] arr, int num,int start,int end) {
        if (arr.length == 0 || end<start)
            return "Eroor";
    
        int mid = (start + end) / 2;
        if (num==arr[mid]) {
            return  "所查数在数组的下标为"+mid;
        }else if(num<arr[mid]) {
            return binarySearch(arr, num,start,mid-1);
        }else {
            return binarySearch(arr, num, mid+1, end);
        }
}