Playing Piano CodeForces - 1079C

http://codeforces.com/contest/1079/problem/C

按贪心构造想了半天 看标签是dp。。

b[i]只和a[i-1] b[i-1]和a[i+1] b[i+1]有关 用dp[i][j]代表i位置放j是否可行 pre[i][j]记录下路径

#include <bits/stdc++.h>
using namespace std;
const int maxn=1e5+10;

int dp[maxn][6],pre[maxn][6];
int ary[maxn],ans[maxn];
int n;

int main()
{
    int i,j,k,p,flag;
    scanf("%d",&n);
    for(i=1;i<=n;i++) scanf("%d",&ary[i]);
    for(j=1;j<=5;j++) dp[1][j]=1;
    for(i=2;i<=n;i++){
        for(j=1;j<=5;j++){
            for(k=1;k<=5;k++){
                if(ary[i-1]<ary[i]){
                    if(k<j&&dp[i-1][k]){
                        dp[i][j]=1,pre[i][j]=k;
                        break;
                    }
                }
                else if(ary[i-1]==ary[i]){
                    if(k!=j&&dp[i-1][k]){
                        dp[i][j]=1,pre[i][j]=k;
                        break;
                    }
                }
                else{
                    if(k>j&&dp[i-1][k]){
                        dp[i][j]=1,pre[i][j]=k;
                        break;
                    }
                }
            }
        }
    }
    flag=0;
    for(j=1;j<=5;j++){
        for(i=n,p=j;i>=1&&dp[i][p]!=0;p=pre[i][p],i--) ans[i]=p;
        if(i==0){
            flag=1;
            break;
        }
    }
    if(flag){
        for(i=1;i<=n;i++) printf("%d ",ans[i]);
        printf("\n");
    }
    else printf("-1\n");
    return 0;
}