hdu 2689 Sort it(线段树)

来源：互联网发布：淘宝网购物如何退货编辑：程序博客网时间：2024/05/15 23:52

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;}

0 0