【USACO】Sorting a Three-Valued Sequence（思路）

来源：互联网发布：js跨域请求编辑：程序博客网时间：2024/06/07 03:28

拼了老命用一种贪心的思想把它A了，但是代码写的太烂了，而且时间复杂度为 n ^ 2，我就不多说了，太烂了

之后上网找了一个规律，时间复杂度为 nlogn，而且思路很明确，又写了一遍

代码：（贪心）

/*ID: 18906421LANG: C++PROG: sort3*/#include<cstdio>#include<cstring>#include<vector>#include<algorithm>using namespace std;const int maxn = 1005;int n;int arr[maxn];int sorted[maxn];int vis[4] = {0};int s[3];int find_pos(int u,int v,int num){    int re_pos;    for(int i = 0; i < num; i++){        if(arr[i] == v){            if(i >= s[u])                return i;            else                re_pos = i;        }    }    return re_pos;}int main(){    int n,cnt = 0,pos;    freopen("sort3.in","r",stdin);    freopen("sort3.out","w",stdout);    scanf("%d",&n);    for(int i = 0; i < n; i ++){        scanf("%d",&arr[i]);        sorted[i] = arr[i];        vis[arr[i]]++;    }    s[1] = -1; s[2] = -1;    sort(sorted,sorted + n);    for(int i = 0; i < n; i++)        if(sorted[i] == 1 && s[1] < 0) s[1] = i;        else if(sorted[i] == 2 && s[2] < 0) s[2] = i;    //printf("%d %d\n",s[1],s[2]);    for(int i = n - 1; i >= 0; i--){        //printf("%d %d %d\n",vis[1],vis[2],vis[3]);        if(arr[i] == 1){            if(vis[3]){                pos = find_pos(1,3,i);                //printf("%d %d\n",i,pos);                arr[pos] = 1;                cnt ++;                vis[3] --;            }            else if(vis[2]){                pos = find_pos(1,2,i);                //printf("%d %d\n",i,pos);                arr[pos] = 1;                cnt ++;                vis[2] --;            }            else                vis[1] --;        }        else if(arr[i] == 2){            if(vis[3]){                pos = find_pos(2,3,i);                //printf("%d %d\n",i,pos);                arr[pos] = 2;                cnt ++;                vis[3] --;            }            else vis[2] --;        }        else            vis[3] --;    }    printf("%d\n",cnt);    return 0;}

思路：

/*ID: 18906421LANG: C++PROG: sort3*/#include<cstdio>#include<cstring>#include<vector>#include<algorithm>using namespace std;const int maxn = 1005;int n,ans;int arr[maxn];int sorted[maxn];int vis[4][4] = {0};int main(){    freopen("sort3.in","r",stdin);    freopen("sort3.out","w",stdout);    int n;    scanf("%d",&n);    for(int i = 0; i < n; i++){        scanf("%d",&arr[i]);        sorted[i] = arr[i];    }    sort(sorted,sorted + n);    for(int i = 0; i < n; i++){        if(arr[i] != sorted[i])            vis[sorted[i]][arr[i]] ++;    }    int v1 = min(vis[1][2],vis[2][1]);    vis[1][2] -= v1; vis[2][1] -= v1;    int v2 = min(vis[1][3],vis[3][1]);    vis[1][3] -= v2; vis[3][1] -= v2;    int v3 = min(vis[2][3],vis[3][2]);    vis[2][3] -= v3; vis[3][2] -= v3;    ans = v1 + v2 + v3;    ans += (vis[1][2] + vis[2][1] + vis[1][3] + vis[3][1] + vis[2][3] + vis[3][2]) / 3 * 2;    printf("%d\n",ans);    return 0;}

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