HDU 1394（线段树）

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1394

区间求和问题，但是用暴力方法也能通过，线段树50MS可以解决，暴力要250多MS，这就是算法的魅力所在~~~

线段树代码：

#include <stdio.h>
#define L(x) ((x) << 1)
#define R(x) ((x) << 1 | 1)
#define N 5001

typedef struct
{
    int lson, rson;
    int val;
}   seg_tree;
seg_tree s[N << 2];
int num[N];

void build(int left, int right, int idx)
{
    s[idx].lson = left;
    s[idx].rson = right;
    s[idx].val = 0;
    if(left == right)
    {
        return;
    }
    int mid = (left + right) / 2;
    build(left, mid, L(idx));
    build(mid + 1, right, R(idx));
}

void update(int id, int idx)
{
    s[idx].val++;
    if(s[idx].lson == s[idx].rson)
    {
        return;
    }
    int mid = (s[idx].lson + s[idx].rson) / 2;
    if(id <= mid)
    {
        update(id, L(idx));
    }
    else
    {
        update(id, R(idx));
    }
}

int query(int left, int right, int idx)
{
    if(left == s[idx].lson && s[idx].rson == right)
    {
        return s[idx].val;
    }
    int mid = (s[idx].lson + s[idx].rson) / 2;
    if(right <= mid)
    {
        return query(left, right, L(idx));
    }
    else if(left > mid)
    {
        return query(left, right, R(idx));
    }
    else
    {
        return query(left, mid, L(idx)) + query(mid + 1, right, R(idx));
    }
}

int main()
{
    int n;
    while(scanf("%d", &n) != EOF)
    {
        int i;
        int sum = 0;
        build(1, n, 1);
        for(i = 1; i <= n; i++)
        {
            scanf("%d", &num[i]);
            sum += query(num[i] + 1, n, 1);
            update(num[i] + 1, 1);
        }
        int res = sum;
        for(i = 1; i <= n; i++)
        {
            sum = sum - num[i] + (n - num[i] - 1);
            if(res > sum)
                res = sum;
        }
        printf("%d\n", res);
    }
    return 0;
}

暴力求解代码：

#include <stdio.h>
#include <string.h>
#define N 5001

int a[N];
int b[N];

int main()
{
    int n;
    while(scanf("%d", &n) != EOF)
    {
        memset(b, 0, sizeof(b));
        int i, j, t = 0;
        for(i = 0; i < n; i++)
        {
            scanf("%d", &a[i]);
        }
        for(i = 0; i < n; i++)
        {
            for(j = i + 1; j < n; j++)
            {
                if(a[i] > a[j])
                {
                    b[i]++;
                }
            }
            t += b[i];
        }
        int min = t;
        for(i = 0; i < n; i++)
        {
            t = t - a[i] + (n - a[i] - 1);
            if(min > t)
            {
                min = t;
            }
        }
        printf("%d\n", min);
    }
}