Manthan, Codefest 19 (open for everyone, rated, Div. 1 + Div. 2) D. Restore Permutation（树状数组+二分）

题目链接


思路：这题明着想确实不好想，不过倒着想其实挺简单的，用树状数组维护前缀和，要想知道第i位填的是哪些数字，只要二分一下大于a【i】的前缀和，r就是答案，然后依次动态更新。

#include<bits/stdc++.h>
#define lowbit(i)  (i)&(-i)
#define ll long long
using namespace std;
const int maxn=2e5+10;
ll a[maxn],c[maxn],ans[maxn];
void add(ll x,ll k)
{
	while(x<maxn) c[x]+=k,x+=lowbit(x);
}
ll query(ll x)
{
	ll res=0;
	while(x>0) res+=c[x],x-=lowbit(x);
	return res;
}
int main()
{
	int n;
	scanf("%d",&n);
	for(int i=1;i<=n;++i) scanf("%lld",&a[i]),add(i,i);
	for(int i=n;i>=1;--i)
	{
		ll l=1,r=n;
		while(l<=r)
		{
			ll mid=(l+r)>>1;
			if(query(mid-1)<=a[i]) l=mid+1;
			else r=mid-1;
		}
		ans[i]=r;
		add(r,-r); 
	}
	for(int i=1;i<=n;++i) printf("%lld ",ans[i]);
}