2805_inv_线段树

Posted 2020-10-12

题目描述

给定N，以及A1，A2，……AN，求所有的数对（i，j）同时满足： 
（1）i＜j 
（2）2Ai＞Aj

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思路

对于逆序对有显然的线段树做法

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#include <stdio.h>
#define N 4000001
#define LL long long
LL t[N];
int a[N];
int insert(int p, int l, int r, int x)
{
    if (l == r)
    {
        t[p] = 1;
        return 0;
    }
    int mid = (l + r) >> 1;
    if (x <= mid) insert(p << 1, l, mid, x);
    else insert(p << 1 | 1, mid + 1, r, x);
    t[p] = t[p << 1] + t[p << 1 | 1];
}
LL find(int p, int l, int r, int x, int y)
{
    if (x <= l && y >= r)
        return t[p];
    int mid = (l + r) >> 1;
    if (y <= mid) return find(p << 1, l, mid, x, y);
    else if (x > mid) return find(p << 1 | 1, mid + 1, r, x, y);
    else return find(p << 1, l, mid, x, mid) + find(p << 1 | 1, mid + 1, r, mid + 1, y);
}
int main()
{
    int n;
    scanf("%d", &n);
    LL ans = 0;
    for (int i = 1; i <= n; i++)
    {
        scanf("%d", &a[i]);
        ans += find(1, 1, 10 * n, a[i] / 2 + 1, 10 * n);
        insert(1, 1, 10 * n, a[i]);
    }
    printf("%lld\n", ans);
}