一开始看错题意了！！没注意到整个数列是1~n的一个排列！
很水的一道题，找到b在数列中的位置设为point，比b大的赋值为-1，比b小的赋值为1；
然后求出sum[i,point]的值出现了几次记为lfre[sum[i,point]]++; ans += lfre[sum[i,point]]*rfre[-sum[i,point]];
由于c++数组不能是负数，所以稍微处理一下
1 /* 2 ID:WULALA 3 PROB:bzoj1303 4 LANG:C++ 5 */ 6 #include <cstdio> 7 #include <cstring> 8 #include <algorithm> 9 #include <cmath> 10 #include <iostream> 11 #include <fstream> 12 #include <ctime> 13 #define N 100008 14 #define M 15 #define mod 16 #define mid(l,r) ((l+r) >> 1) 17 #define INF 0x7ffffff 18 using namespace std; 19 20 int b,point,n,num[N],sum[N],lfre[2 * N],rfre[2 * N],ans; 21 22 int main() 23 { 24 scanf("%d%d",&n,&b); 25 for (int i = 1;i <= n;i++) 26 { 27 scanf("%d",&num[i]); 28 if (num[i] < b) num[i] = -1; 29 else if (num[i] > b) num[i] = 1; 30 else if (num[i] == b) point = i,num[i] = 0; 31 } 32 lfre[n] = 1; rfre[n] = 1; 33 for (int i = point - 1;i >= 1;i--) sum[i] = sum[i+1] + num[i],lfre[sum[i]+n]++; 34 for (int i = point + 1;i <= n;i++) sum[i] = sum[i-1] + num[i],rfre[sum[i]+n]++; 35 for (int i = 0;i <= 2 * n - 1;i++) ans += lfre[i] * rfre[2*n-i]; 36 printf("%d ",ans); 37 return 0; 38 }