# 二分查找(等于x，小于x，小于等于x，大于x，大于等于x )

2018/05/25 08:37

``//等于x//小于x//小于等于x//大于x//大于等于x``
`````` 1 #include <cstdio>
2 #include <cstdlib>
3 #include <cmath>
4 #include <cstring>
5 #include <string>
6 #include <set>
7 #include <map>
8 #include <list>
9 #include <stack>
10 #include <queue>
11 #include <vector>
12 #include <algorithm>
13 #include <iostream>
14 using namespace std;
15 #define ll long long
16 #define minv 1e-6
17 #define inf 1e9
18 const long maxn=1e5+5;
19 const ll mod=1e9+7;
20
21 //µÈÓÚx
22 //Ð¡ÓÚx
23 //Ð¡ÓÚµÈÓÚx
24 //´óÓÚx
25 //´óÓÚµÈÓÚx
26
27 long a[maxn];
28
29 int main()
30 {
31     long n,s,i,l,r,m;
32     scanf("%ld",&n);
33     for (i=1;i<=n;i++)
34         scanf("%ld",&a[i]);
35     scanf("%ld",&s);
36     l=1; r=n;
37     while (l<=r)
38     {
39         m=(l+r)>>1;
40         if (a[m]>=s) //a[l]>=s
41             r=m-1; //a[r]<s
42         else
43             l=m+1;
44     }
45     printf("%ld\n",a[l]);
46     printf("%ld\n",a[r]);
47     /*
48         8 1 1 1 4 4 6 6 6
49         0
50         1 0
51
52         8 1 1 1 4 4 6 6 6
53         10
54         0 6
55
56         8 1 1 1 4 4 6 6 6
57         4
58         4 1
59
60
61         8 1 1 1 4 4 6 6 6
62         3
63         4 1
64
65         8 1 1 1 4 4 6 6 6
66         5
67         6 4
68
69
70
71
72     */
73     return 0;
74 }``````

a[l]>s a[r]<=s

`````` 1 #include <cstdio>
2 #include <cstdlib>
3 #include <cmath>
4 #include <cstring>
5 #include <string>
6 #include <set>
7 #include <map>
8 #include <list>
9 #include <stack>
10 #include <queue>
11 #include <vector>
12 #include <algorithm>
13 #include <iostream>
14 using namespace std;
15 #define ll long long
16 #define minv 1e-6
17 #define inf 1e9
18 const long maxn=1e5+5;
19 const ll mod=1e9+7;
20
21 //µÈÓÚx
22 //Ð¡ÓÚx
23 //Ð¡ÓÚµÈÓÚx
24 //´óÓÚx
25 //´óÓÚµÈÓÚx
26
27 long a[maxn];
28
29 int main()
30 {
31     long n,s,i,l,r,m;
32     scanf("%ld",&n);
33     for (i=1;i<=n;i++)
34         scanf("%ld",&a[i]);
35     scanf("%ld",&s);
36     l=1; r=n;
37     while (l<=r)
38     {
39         m=(l+r)>>1;
40         if (a[m]>s) //a[l]>s
41             r=m-1; //a[r]<=s
42         else
43             l=m+1;
44     }
45     printf("%ld\n",a[l]);
46     printf("%ld\n",a[r]);
47     /*
48         8 1 1 1 4 4 6 6 6
49         0
50         1 0
51
52         8 1 1 1 4 4 6 6 6
53         10
54         0 6
55
56         8 1 1 1 4 4 6 6 6
57         4
58         6 4
59
60
61         8 1 1 1 4 4 6 6 6
62         3
63         4 1
64
65         8 1 1 1 4 4 6 6 6
66         5
67         6 4
68
69
70
71
72     */
73     return 0;
74 }``````

Code1 ： if l!=n+1 && a[l]==s

Code2 ： if r!=0 && a[r]==s

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