【USACO】 Balanced Photo

【题目链接】

           点击打开链接

【算法】

         树状数组

【代码】

        

#include<bits/stdc++.h>

using namespace std;

int i,N,ans,l1,l2;
int c[100005],val[100005],id[100005];

template <typename T> void read(T &x) {
        int f=1; char c = getchar(); x=0;
        for (; !isdigit(c); c = getchar()) { if (c=='-') f=-1; }
        for (; isdigit(c); c = getchar()) x=x*10+c-'0';
        x*=f;
}

bool cmp(int a,int b) {
        return val[a] > val[b];
}

int lowbit(int x) {
        return x & -x;
}

int query(int x) {
        int sum=0;
        while (x >= 1) {
              sum += c[x];
              x -= lowbit(x);
        }    
        return sum;
}

void change(int x,int y) {
        while (x <= N) {
                c[x] += y;
                x += lowbit(x);
        }    
}

int main() {
        
        scanf("%d",&N);
        for (i = 1; i <= N; i++) {
                scanf("%d",&val[i]);
                id[i] = i;
        }
        sort(id+1,id+N+1,cmp);
        
        for (i = 1; i <= N; i++) {
                l1 = query(id[i]-1);
                l2 = i - l1 - 1;
                if (max(l1,l2) > min(l1,l2) * 2) ans++;
                change(id[i],1);    
        }
        
        printf("%d\n",ans);
        
        return 0;
    
}