CF 159 div2 d

a[i] <= a[i+1] <= 2*a[i] 可得,0 <= a[n] - a[n-1] <= a[n-1],这样记a[n]-a[n-1] 为tem,
则 若tem-a[n-2] >= 0,则令tem = tem - a[n-2],此时 0 <= tem <= a[n-2](a[n-1] - a[n-2] <= a[n-2]),
若 tem-a[n-2] < 0 ,则令tem = a[n-2] - tem,此时 0 <= tem <= a[n-2],这样不停的维护使 0 <= tem <= a[i]
,最后直到a[1] 都满足,解就构造出来了。 

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;
typedef long long LL;
const int maxn = 100000 + 5;
const int INF = 1000000000;LL a[maxn];
int ans[maxn];
int tag[maxn];int main(){int n;while(cin >> n){for(int i = 0;i < n;i++) cin >> a[i];LL tem = a[n-1] - a[n-2];ans[n-1] = 1;ans[n-2] = 0;memset(tag,0,sizeof(tag));for(int i = n - 3;i >= 0;i--){if(tem - a[i] >= 0){ans[i] = 0;tem = tem - a[i];}else{ans[i] = 1;tag[i] = 1;tem = a[i] - tem;}}int cnt = 0;for(int i = 0;i < n;i++){if((ans[i]+cnt)%2 == 1) cout << '+';else cout << '-';cnt += tag[i];}cout << endl;}return 0;
}


 


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