hdoj 6058(2017 Multi-University Training Contest
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题目链接:Kanade’s sum
题目大意:给你一个n个数的数列,数列是n个数的某一个排列,枚举l,r,每个[l,r]区间里面第k大的数作为这个贡献,也就是算
题目思路:暴力肯定T,没得说,因为是一个全排列,我们可以想到去算每个数是第k大的时候有多少次,然后这个东西怎么去算呢?我们可以去找每个数前面最近的比他大的k个数和后面比他大的k个数,然后算贡献就好了,怎么算,前面枚举i个数,那么后面就应该是k-i个数,然后我们如果一直枚举的话,会重复,所以我们需要减掉重复,这就是代码里面“(a[i]-a[i+1])*(b[k-i+2]-b[k-i+1]);”的作用,去算次数,而且不重复,好了,现在我们知道怎么去算贡献了,现在我们需要去做的就是简化这个过程:首先我们可以想到,因为每个数都是唯一的,所以我们可不可以从最小的开始枚举,这样枚举大的数的时候这个小数只会影响贡献不会影响他是第几大,而贡献我们可以用链表去维护,先给定一个满链表,然后删去小的,算贡献(代码不懂的,可以打印a[i],b[i]每次存的数和每次的贡献,应该就能懂了)
#include <map>#include <set>#include <cmath>#include <queue>#include <stack>#include <vector>#include <cstdio>#include <cstring>#include <cstdlib>#include <iostream>#include <algorithm>using namespace std;typedef long long ll;const int maxn = 5e5+10;int pos[maxn],pre[maxn],nxt[maxn];ll a[maxn],b[maxn];int n,k;ll solve(int x){ int t1 = 0,t2 = 0; ll ans = 0; for(int i = x;i >= 0&&t1 <= k;i = pre[i]) a[++t1] = i; for(int i = x;i <= n+1&&t2 <= k;i = nxt[i]) b[++t2] = i; for(int i = 1;i <= t1-1;i++){ if(k-i+2 > t2) continue; ll tmp = (a[i]-a[i+1])*(b[k-i+2]-b[k-i+1]); ans += tmp; } return ans;}void del(int x){ nxt[pre[x]] = nxt[x]; pre[nxt[x]] = pre[x];}int main(){ int T; scanf("%d",&T); while(T--){ scanf("%d%d",&n,&k); for(int i = 1;i <= n;i++){ scanf("%d",&a[i]); pos[a[i]] = i; pre[i] = i-1; nxt[i] = i+1; } pre[0] = -1;nxt[n+1] = n+2; ll ans = 0; for(int i = 1;i <= n;i++){ ans += solve(pos[i])*i; del(pos[i]); } printf("%lld\n",ans); } return 0;}
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