....最后想到了，可是不应该枚举a[]，这样要二重循环，而应该枚举b[],这样只需一重循环。。。
1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 using namespace std; 6 typedef long long ll; 7 const int maxn=1e5+10; 8 int a[maxn],b[maxn],c[maxn]; 9 int n; 10 11 int FindLastSmaller(int key) 12 { 13 int l=0,r=n-1; 14 while(l<=r){ 15 int mid=(l+r)>>1; 16 if(a[mid]>=key) r=mid-1; 17 else l=mid+1; 18 } 19 return r; 20 } 21 22 int FindFirstLarger(int key) 23 { 24 int l=0,r=n-1; 25 while(l<=r){ 26 int mid=(l+r)>>1; 27 if(c[mid]>key) r=mid-1; 28 else l=mid+1; 29 } 30 return l; 31 } 32 33 int main() 34 { 35 cin>>n; 36 for(int i=0;i<n;i++) cin>>a[i]; 37 for(int i=0;i<n;i++) cin>>b[i]; 38 for(int i=0;i<n;i++) cin>>c[i]; 39 sort(a,a+n); 40 sort(b,b+n); 41 sort(c,c+n); 42 ll ans=0; 43 for(int i=0;i<n;i++){ 44 ll p1=FindLastSmaller(b[i]); 45 ll p2=FindFirstLarger(b[i]); 46 ans+=(p1+1)*(n-p2); 47 } 48 cout<<ans<<endl; 49 return 0; 50 }
lower_bound(),upper_bounder()不能再好用。。。
1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 using namespace std; 6 typedef long long ll; 7 const int maxn = 1e5 + 1000; 8 int a[maxn], b[maxn], c[maxn]; 9 int n; 10 11 int main() 12 { 13 scanf("%d", &n); 14 for (int i = 0; i < n; i++) scanf("%d", &a[i]); 15 for (int i = 0; i < n; i++) scanf("%d", &b[i]); 16 for (int i = 0; i < n; i++) scanf("%d", &c[i]); 17 sort(a, a + n); 18 sort(b, b + n); 19 sort(c, c + n); 20 ll ans = 0; 21 for (int i = 0; i<n; i++) { 22 ll p = lower_bound(a, a + n, b[i]) - a; 23 ll q = upper_bound(c, c + n, b[i]) - c; 24 ans += p*(n - q); 25 } 26 printf("%lld ", ans); 27 }