二分查找(等于x，小于x，小于等于x，大于x，大于等于x )

//等于x
//小于x
//小于等于x
//大于x
//大于等于x

 #include <cstdio>
 #include <cstdlib>
 #include <cmath>
 #include <cstring>
 #include <string>
 #include <set>
 #include <map>
 #include <list>
 #include <stack>
 #include <queue>
 #include <vector>
 #include <algorithm>
 #include <iostream>
 using namespace std;
 #define ll long long
 #define minv 1e-6
 #define inf 1e9
 const long maxn=1e5+;
 const ll mod=1e9+;

 //µÈÓÚx
 //СÓÚx
 //СÓÚµÈÓÚx
 //´óÓÚx
 //´óÓÚµÈÓÚx 

 long a[maxn];

 int main()
 {
     long n,s,i,l,r,m;
     scanf("%ld",&n);
     for (i=;i<=n;i++)
         scanf("%ld",&a[i]);
     scanf("%ld",&s);
     l=; r=n;
     while (l<=r)
     {
         m=(l+r)>>;
         if (a[m]>=s) //a[l]>=s
             r=m-; //a[r]<s
         else
             l=m+;
     }
     printf("%ld\n",a[l]);
     printf("%ld\n",a[r]);
     /*
         8 1 1 1 4 4 6 6 6
         0
         1 0

         8 1 1 1 4 4 6 6 6
         10
         0 6

         8 1 1 1 4 4 6 6 6
         4
         4 1
         

         8 1 1 1 4 4 6 6 6
         3
         4 1

         8 1 1 1 4 4 6 6 6
         5
         6 4

     */
     return ;
 }

　　a[l]>s a[r]<=s

 #include <cstdio>
 #include <cstdlib>
 #include <cmath>
 #include <cstring>
 #include <string>
 #include <set>
 #include <map>
 #include <list>
 #include <stack>
 #include <queue>
 #include <vector>
 #include <algorithm>
 #include <iostream>
 using namespace std;
 #define ll long long
 #define minv 1e-6
 #define inf 1e9
 const long maxn=1e5+;
 const ll mod=1e9+;

 //µÈÓÚx
 //СÓÚx
 //СÓÚµÈÓÚx
 //´óÓÚx
 //´óÓÚµÈÓÚx 

 long a[maxn];

 int main()
 {
     long n,s,i,l,r,m;
     scanf("%ld",&n);
     for (i=;i<=n;i++)
         scanf("%ld",&a[i]);
     scanf("%ld",&s);
     l=; r=n;
     while (l<=r)
     {
         m=(l+r)>>;
         if (a[m]>s) //a[l]>s
             r=m-; //a[r]<=s
         else
             l=m+;
     }
     printf("%ld\n",a[l]);
     printf("%ld\n",a[r]);
     /*
         8 1 1 1 4 4 6 6 6
         0
         1 0

         8 1 1 1 4 4 6 6 6
         10
         0 6

         8 1 1 1 4 4 6 6 6
         4
         6 4
         

         8 1 1 1 4 4 6 6 6
         3
         4 1

         8 1 1 1 4 4 6 6 6
         5
         6 4

     */
     return ;
 }

若要判断相等，则

Code1 ： if l!=n+1 && a[l]==s

Code2 ： if r!=0 && a[r]==s