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

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: 18906421
LANG: 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: 18906421
LANG: 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;
}