二分查找——有序数组

折半查找法，每次都是去找最中间的数，前提是这个数组必须是有序的，从小到大

 

JAVA实现 

 public static void main(String[] args) {
        int []arr={1,2,3,4,5,6,7,8};
        System.out.println(find_index(arr,3));
        System.out.println(find_index(arr,8));
        System.out.println(find_index(arr,9));
    }

    /**
     * 二分查找的实现
     * @param arr
     * @param num
     * @return
     */
    public static int find_index(int []arr,int num){
        int left=0;//定义左边界
        int right=arr.length-1;//定义右边界
        while (left<=right){
            int mid=(left+right)/2;
            if (num>arr[mid]){
                left=mid+1;
            }
            else if (num

C语言实现

#include
#define N 100
int main()//二分法前提查找的数据是有序的
{
    int arr[N] = { 0 };
    for (int i = 0; i < N; i++)
    {
        arr[i] = i;
    }
    int n=0;
    scanf("%d", &n);
    int right = 99;
    int left = 0;
    while (left<=right)
    {    int mid = (right + left) / 2;
         if (arr[mid] == n)
            {    printf("找到了，下标是%d", mid);
                 break;
            }
         if (arr[mid]>n)
            {
            right = mid-1;
            }
         if (arr[mid]right){printf("该数字不存在");}
        return 0;
}