51Nod 1001数组中和等于K的数对

Input示例

8 9
-1
6
5
3
4
2
9
0
8

Output示例

-1 9
0 8
2 6
3 5

first try:

#include "bits/stdc++.h"
using namespace std;
#define LL long long
#define INF 0x3f3f3f3f3f
#define PI acos(-1)
#define N 50010
int arr[N];
int main()
{
    int n,k;
    while(~scanf("%d%d",&k,&n)){
        for(int i=0;i<n;i++){
            scanf("%d",&arr[i]);
        }
        sort(arr,arr+n);
        int c=0;
        for(int i=0;i<n;i++){
            for(int j=i+1;j<n;j++){
                if(arr[i]+arr[j]==k){
                    printf("%d %d
",arr[i],arr[j]);
                    c++;
                }
            }
        }
        if(!c){
            printf("No Solution
");
        }
    }
    return 0;
}

O(n^2)

Time limit exceeded，优化

 second try:

第二层从后向前找，及时跳出

#include "bits/stdc++.h"
using namespace std;
#define LL long long
#define INF 0x3f3f3f3f3f
#define PI acos(-1)
#define N 50010
int arr[N];
int main()
{
    int n,k;
    while(~scanf("%d%d",&k,&n)){
        for(int i=0;i<n;i++){
            scanf("%d",&arr[i]);
        }
        sort(arr,arr+n);
        int c=0;
        for(int i=0;i<n;i++){
            for(int j=n-1;j>=i+1;j--){
                if(arr[i]+arr[j]==k){
                    printf("%d %d
",arr[i],arr[j]);
                    c++;
                    break;
                }
            }
        }
        if(!c){
            printf("No Solution
");
        }
    }
    return 0;
}

Time limit exceeded，再优化

third try:

转换思维，查找互补数是否存在，二分查找，分治

#include "bits/stdc++.h"
using namespace std;
#define LL long long
#define INF 0x3f3f3f3f3f
#define PI acos(-1)
#define N 50010
int arr[N];
int Find(int n,int x){
    int r=n-1,l=0,m;
    while(l<=r){
        m=(r+l)/2;
        if(x==arr[m])
            return m;
        else if(x>arr[m])
            l=m+1;
        else
            r=m-1;
    }
    return -1;
}
int main()
{
    int n,k;
    while(~scanf("%d%d",&k,&n)){
        for(int i=0;i<n;i++){
            scanf("%d",&arr[i]);
        }
        sort(arr,arr+n);
        int c=0,tt;
        for(int i=0;i<n;i++){
            tt=Find(n,k-arr[i]);
            if(tt!=-1){
                if(tt<=i)
                    break;
                printf("%d %d
",arr[i],k-arr[i]);
                c++;
            }
        }
        if(!c){
            printf("No Solution
");
        }
    }
    return 0;
}