利用二分法查找有序数组中某一元素

1.利用二分法查找有序数组中某一元素，若有，返回下标，若没有，返回-1。

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

//int binary_search(int* a, int k)
int binary_search(int a[], int k, int sz)
{
 int left = 0;
 int right = sz - 1;

 while (left <= right)
 {
  int mid = right - (right - left) / 2;
  if (a[mid] == k)
  {
   return mid;
  }
  else if (a[mid] > k)
  {
   right = mid - 1;
  }
  else
  {
   left = mid + 1;
  }
 }
 return -1;
}
int main()
{
 int arr[] = { 1,2,3,4,5,6,7,8,9 };
 int key = 6;
 int sz = sizeof(arr) / sizeof(arr[0]);
 int ret = binary_search(arr, key, sz);

 if (-1 == ret)
  printf("找不到\n");
 else
  printf("找到了:%d\n", ret);
 system("pause");
 return 0;
}

2.猜数字游戏

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

void menu()
{
 printf("********   1.play   0.exit    ********\n");
}
void game()
{
 int num = 0;
 int input = 0;
 num = rand()%100+1;
 //printf("%d\n", num);
 while (1)
 {
  printf("请猜数字:>");
  scanf_s("%d", &input);
  if (num == input)
  {
   printf("恭喜你，猜对了\n");
   break;
  }
  else if (input < num)
  {
   printf("猜小了\n");
  }
  else
  {
   printf("猜大了\n");
  }
 }
}
int main()
{
 srand((unsigned int)time(NULL));
 int input = 0;
 do
 {
  menu();
  printf("请选择:>");
  scanf_s("%d", &input);
  switch (input)
  {
  case 1:
   game();
    break;
  case 0:
   printf("退出游戏\n");
   break;
  default:
   printf("选择错误\n");
   break;
  }
 }
 while (input);
return 0;
}

3.模拟三次输入密码的场景

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
 int time = 3;
 char password1[] = "123456";
 char password2[10] = {0};
 while(time)
 {
  printf("请输入密码:>");
  scanf_s("%s", &password2);
  if (strcmp(password1, password2) == 0)
  {
   printf("登录成功\n");
   break;
  }
  else
  {
   ;
  }
  time--;
 }
 if (time == 0)
 {
  printf("密码已输入三次错误，退出登录\n");
 }
 system("pause");
 return 0;
}