hdu 2689 Sort it(线段树)

题目链接：hdu 2689 Sort it

题目大意：求逆序对数。

解题思路：用水水的线段树做了一下。

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

const int maxn = 1005;

#define lson(x) ((x)<<1)
#define rson(x) (((x)<<1)|1)
int lc[maxn << 2], rc[maxn << 2], s[maxn << 2];

inline void pushup(int u) {
    s[u] = s[lson(u)] + s[rson(u)];
}

void build (int u, int l, int r) {
    lc[u] = l;
    rc[u] = r;
    s[u] = 0;

    if (l == r) 
        return;

    int mid = (l + r) >> 1;
    build(lson(u), l, mid);
    build(rson(u), mid + 1, r);
    pushup(u);
}

void modify(int u, int x, int w) {
    if (x == lc[u] && rc[u] == x) {
        s[u] += w;
        return;
    }

    int mid = (lc[u] + rc[u]) >> 1;
    if (x <= mid)
        modify(lson(u), x, w);
    else 
        modify(rson(u), x, w);
    pushup(u);
}

int query(int u, int l, int r) {
    if (l <= lc[u] && rc[u] <= r)
        return s[u];

    int mid = (lc[u] + rc[u]) >> 1, ret = 0;
    if (l <= mid)
        ret += query(lson(u), l, r);
    if (r > mid)
        ret += query(rson(u), l, r);
    return ret;
}

int N, arr[maxn];

int main () {
    while (scanf("%d", &N) == 1) {
        build (1, 0, 1000);
        for (int i = 1; i <= N; i++)
            scanf("%d", &arr[i]);

        int ans = 0;
        for (int i = N; i; i--) {
            ans += query(1, 0, arr[i]);
            modify(1, arr[i], 1);
        }
        printf("%d\n", ans);
    }
    return 0;
}