poj 2299

之前用树状数组做这道题，没发现有什么问题。用线段数组后，总是报 runtime Error。后来发现是数组开小了。

# include <iostream>
# include <algorithm>
# include <memory.h>
# include <vector>

using namespace std;

const int maxn = 500010;

struct Num {
  int val;
  int pos;
  bool operator <(const Num &a)const {
    return val < a.val;
  }
}num[maxn];

int n;



struct Segment {
  // 因为不需要修改
  struct _nod{
    long long nums;
    int l, r;
  } TreeNode[maxn << 2]; //bug 之前开到 [maxn <<1] 直接报 runtime error。 线段树结构占用内存是树状数组的 4 倍，而且查询数据还没有树状数组快！ 
  
  void init() {
    memset(TreeNode, 0, sizeof(TreeNode));
  }

  void build(int index, int l, int r) {
    int mid = (l+r) >> 1;
    TreeNode[index].l = l; TreeNode[index].r = r;
    if (l == r)
      return;
    build(index <<1, l, mid); build(index << 1|1, mid+1, r);
  }
  
  void add_node(int index, int pos, int l, int r) {
    int mid = (l+r) >> 1;
    if ( l == r) {
      TreeNode[index].nums = 1;
      return;
    }
    if (mid >= pos) {
      add_node(index <<1, pos, l, mid); 
    }
    else {
      add_node(index <<1|1, pos, mid+1, r); 
    }
    TreeNode[index].nums = TreeNode[index<<1].nums + TreeNode[index<<1|1].nums;
  }
  
  long long query(int index, int l, int r) {
    long long sums = 0;
    int mid = 0;
    if (l== TreeNode[index].l and r == TreeNode[index].r)
       return TreeNode[index].nums;
    
    mid = (TreeNode[index].l + TreeNode[index].r) >> 1;
    if (mid >= r)
      return query(index<<1, l, r);
    else if ( l> mid)
      return query(index<<1|1, l, r);
    else
       return query(index<<1,l,mid)+ query(index<<1|1,mid+1, r);       
  }
    
}T;


int main(int argc, char* argv[]){
  
  while (cin >> n&&n) {
    long long  ans = 0;
    T.init();
    if (1 > n)
      break;
    for (int i=0; i<n; i++){
      cin >> num[i].val;
      num[i].pos = i;
    }
    sort(num, num+n);
    
    T.build(1, 1, n);
    
    for (int i =0; i<n; i++){
      ans += T.query(1, 1, n) - T.query(1, 1, num[i].pos+1);
      T.add_node(1, num[i].pos+1, 1, n);
    }
    printf("%lld
", ans);
  }
  return 0;
}