二分查找 模板

 1 int bsearch(int l, int h, int k)//二分查找函数
 2 {
 3     int i, mid;
 4
 5     while(l<=h){
 6         mid = l+(h-l)/2;
 7         if(X[mid]>k)
 8             h = mid-1;
 9         else if(X[mid]<k)
10             l = mid+1;
11         else
12             break;
13     }
14     return mid;
15 }

 1 int max_bsearch(int l, int h, int k)//求上界
 2 {
 3     int i, mid, ans = -1;
 4
 5     while(l<=h){
 6         mid = l+(h-l)/2;
 7         if(X[mid]>=k){
 8             h = mid-1;
 9             ans =X[mid];
10         }
11         else
12             l = mid+1;
13     }
14     return ans;
15 }

 1 int min_bsearch(int l, int h,int k)//求下界
 2 {
 3     int i, mid, ans = -1;
 4
 5     while(l<=h){
 6         mid = l+(h-l)/2;
 7         if(X[mid]<=k){
 8             l = mid+1;
 9             ans = X[mid];
10         }
11         else
12             h = mid-1;
13     }
14     return ans;
15 }

 1 void Quick_sort(int l, int h)//复习一下快排
 2 {
 3     int i = l, j = h;
 4     int v = X[l];
 5
 6     if(l>=h)
 7         return ;
 8     while(i<j){
 9         while(i<j && X[j]>=v)j--;
10         X[i] = X[j];
11         while(i<j && X[i]<=v)i++;
12         X[j] = X[i];
13     }
14     X[i] = v;
15     Quick_sort(l,i-1);
16     Quick_sort(i+1,h);
17 }

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代码在这

  1 #include <iostream>
  2 #include <cstdio>
  3 #include <algorithm>
  4 #include <cstring>
  5 #define ll long long
  6
  7 using namespace std;
  8
  9 int X[1000003];
 10
 11 void Quick_sort(int l, int h)//复习一下快排
 12 {
 13     int i = l, j = h;
 14     int v = X[l];
 15
 16     if(l>=h)
 17         return ;
 18     while(i<j){
 19         while(i<j && X[j]>=v)j--;
 20         X[i] = X[j];
 21         while(i<j && X[i]<=v)i++;
 22         X[j] = X[i];
 23     }
 24     X[i] = v;
 25     Quick_sort(l,i-1);
 26     Quick_sort(i+1,h);
 27 }
 28
 29 int max_bsearch(int l, int h, int k)//求上界
 30 {
 31     int i, mid, ans = -1;
 32
 33     while(l<=h){
 34         mid = l+(h-l)/2;
 35         if(X[mid]>=k){
 36             h = mid-1;
 37             ans =X[mid];
 38         }
 39         else
 40             l = mid+1;
 41     }
 42     return ans;
 43 }
 44
 45 int bsearch(int l, int h, int k)//二分查找函数
 46 {
 47     int i, mid;
 48
 49     while(l<=h){
 50         mid = l+(h-l)/2;
 51         if(X[mid]>k)
 52             h = mid-1;
 53         else if(X[mid]<k)
 54             l = mid+1;
 55         else
 56             break;
 57     }
 58     return mid;
 59 }
 60
 61 int min_bsearch(int l, int h,int k)//求下界
 62 {
 63     int i, mid, ans = -1;
 64
 65     while(l<=h){
 66         mid = l+(h-l)/2;
 67         if(X[mid]<=k){
 68             l = mid+1;
 69             ans = X[mid];
 70         }
 71         else
 72             h = mid-1;
 73     }
 74     return ans;
 75 }
 76
 77 int main()
 78 {
 79     int n, m, i, j, k, a, b;
 80
 81     while(scanf("%d %d",&n,&m)==2){
 82         for(i=0;i<n;i++)
 83             scanf("%d",X+i);
 84         Quick_sort(0,n-1);
 85         for(i=0;i<m;i++)
 86         {
 87             scanf("%d",&k);
 88             a = min_bsearch(0,n-1,k);
 89             b = max_bsearch(0,n-1,k);
 90             if(a == -1)// 1
 91                 printf("%d\n",b);
 92             else if(b == -1)// 2
 93                 printf("%d\n",a);
 94             else if(a==b)
 95                 printf("%d\n",a);
 96             else if(k-a == b-k)
 97                 printf("%d %d\n",a,b);
 98             else if(k-a>b-k)
 99                 printf("%d\n",b);
100             else
101                 printf("%d\n",a);
102         }
103         //1和2两条语句是非常重要的不然的话可以测试一下 n=1 ,m=5,X[0]=1,k=0这组数据。
104         printf("\n");
105     }
106     return 0;
107 }

Here

当然你可以调用C中的<stdlib.h>中的qsort或C++中的<algorithm>sort。二分查找就用C中的<stdlib.h>中的bsearch。

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