HDU 1394 Minimum Inversion Number 线段树

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

没看到多组输入，WA了一万次。。。。。。

其实很简单，有人暴力过得，我感觉归并排序、二叉排序树求逆序数都可以，但是我没写。

 #include <stdio.h>
 #include <string.h>

 const int MAXN = ;

 struct Tree_Node
 {
     int left, right;
     int num;
 } tree[MAXN<<];

 void build(int left, int right, int step)
 {
     tree[step].left = left;
     tree[step].right = right;
     tree[step].num = ;
     if(left == right)
         return;
     int mid = (left + right) >> ;
     build(left, mid, step<<);
     build(mid+, right, step<<|);
 }

 void insert(int cur, int step)
 {
     if(cur >= tree[step].left && cur <= tree[step].right)
         tree[step].num++;
     if(tree[step].left == tree[step].right)
         return;
     int mid = (tree[step].left + tree[step].right) >> ;
     if(cur <= mid)
         insert(cur, step<<);
     else
         insert(cur, step<<|);
 }

 int query(int left, int right, int step)
 {
     if(tree[step].left == left && tree[step].right == right)
     {
         return tree[step].num;
     }
     int mid = (tree[step].left + tree[step].right) >> ;
     if(right <= mid)
     {
         return query(left, right, step<<);
     }
     else if(left > mid)
     {
         return query(left, right, step<<|);
     }
     else
     {
         return query(left, mid, step<<) + query(mid+, right, step<<|);
     }
 }

 int main()
 {
     int n, x[MAXN];
     while(scanf("%d", &n) != EOF)
     {
         int ans = ;
         build(, n, );
         for(int i = ; i < n; i++)
         {
             scanf("%d", &x[i]);
             ans += query(x[i]+, n, );
             insert(x[i], );
         }
         int tmp = ans;
         for(int i = ; i < n; i++)
         {
             tmp += n - x[i] - x[i] - ;
             if(tmp < ans)ans = tmp;
         }
         printf("%d\n", ans);
     }
     return ;
 }