【CDOJ931】Car race game（树状数组求逆序）

题目连接：http://acm.uestc.edu.cn/#/problem/show/931

OJ评判系统有些坑，不支持__int64以及输出的%I64d大家注意。全开long long也会TLE，比较坑。逆序的基础操作题，不错。

#include <bits/stdc++.h>
#define MAX 100010
using namespace std;

int s[MAX * 10];

struct Node {
    int x, v;
} node[MAX];

bool cmp (Node a, Node b) {
    if (a.x == b.x) {
        return a.v > b.v;
    }
    return a.x < b.x;
}

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

int sum (int p) {
    int ans = 0;
    while (p) {
        ans += s[p];
        p -= lowbit(p);
    }
    return ans;
}

void add (int p, int del) {
    while (p <= 1000000) {
        s[p] += del;
        p += lowbit(p);
    }
}

int main () {
    int n, i;
    while (scanf("%d", &n) != EOF) {
        long long ans = 0;
        for (i = 0; i < n; ++ i) {
            scanf("%d %d", &node[i].x, &node[i].v);
        }
        sort (node, node + n, cmp);
        memset(s, 0, sizeof(s));
        for (i = 0; i < n; ++ i) {
            ans = ans + i - sum(node[i].v);
            add(node[i].v, 1);
        }
        printf ("%lld
", ans);
    }
    return 0;
}