\n
\u7b2c\u4e00\u884c\u4e00\u4e2a\u6574\u6570N\uff08N<=10^5\uff09\u3002
\n\u63a5\u4e0b\u6765\u4e00\u884cN\u4e2a\u6570A[i]\uff08A[i]<=30000\uff09\u3002
\n<\/span><\/p>\n<\/div>\n<\/span>Output<\/span><\/h2>\n\n
\u4e00\u884c\u4e00\u4e2a\u6574\u6570\u3002
\n<\/span><\/p>\n<\/div>\n<\/span>Sample Input<\/span><\/h2>\n10
\n3 5 3 6 3 4 10 4 5 2
\n<\/span><\/div>\n<\/span>Sample Output<\/span><\/h2>\n9
\n<\/span><\/div>\n<\/span>HINT<\/span><\/h2>\n<\/div>\n
<\/span>Source<\/span><\/h2>\n\u5999\u554a\uff0c\u6b63\u5e38\u66b4\u529b\u601d\u60f3\u53ef\u4ee5\u679a\u4e3ei,k\uff0c\u8fd9\u6837\u5c31\u53ef\u4ee5n^2\u89e3\u51b3<\/p>\n
\u5f53\u7136\u6211\u4eec\u53ef\u4ee5\u53d1\u73b0\u8fd9\u4e2a\u5f88\u50cfFFT\uff0c\u4f46\u662f\u4f3c\u4e4e\u590d\u6742\u5ea6O(n^2logn)?<\/p>\n
\u90a3\u4e48\u5982\u4f55\u4f18\u5316<\/p>\n
\u8003\u8651\u5206\u5757\uff0c\u5206\u7c7b\u8ba8\u8bba<\/p>\n
\n- \u5982\u679ci,j,k\u5728\u4e00\u4e2a\u5757\u5185\uff0c\u5c31\u53ef\u4ee5\u76f4\u63a5\u66b4\u529b\uff0c\u590d\u6742\u5ea6O(\u5757\u5927\u5c0f*n)<\/li>\n
- \u5982\u679ci,j\u6216j,k\uff0c\u4ecd\u53ef\u4ee5\u66b4\u529b\uff0c\u5728\u5757\u5185\u679a\u4e3e\uff0c\u9884\u5904\u7406\u524d\u7f00\u4fe1\u606f\uff0c\u590d\u6742\u5ea6\u8fd8\u662fO(\u5757\u5927\u5c0f*n)<\/li>\n
- \u5426\u5219\u5c31\u53ef\u4ee5FFT\uff0c\u679a\u4e3e\u6bcf\u4e00\u5757\uff0c\u5de6\u9762\u4e0e\u53f3\u9762\u5377\u79ef\uff0c\u5728\u4e2d\u95f4\u679a\u4e3ej\uff0c\u52a0\u8d77\u6765\u5c31\u597d\u4e86\uff0c\u590d\u6742\u5ea6O(n^2logn\/\u5757\u5927\u5c0f)<\/li>\n<\/ol>\n
\u56e0\u4e3aFFT\u6709log\uff0c\u6240\u4ee5\u5757\u5927\u5c0f\u5e94\u8be5\u8bbe\u5927\u4e00\u4e9b\uff0c\uff0c\u8bbe\u4e3a2000\u6bd4\u8f83\u5408\u9002\uff1f<\/p>\n
\uff08\u4e0d\u8fc7\u6211\u5e38\u6570\u597d\u5927\u8dd1\u7684\u597d\u6162\u554aqwq<\/p>\n
#include<map>\r\n#include<cmath>\r\n#include<queue>\r\n#include<cstdio>\r\n#include<cstring>\r\n#include<iostream>\r\n#include<algorithm>\r\nusing namespace std;\r\n#define inf 1000000007\r\n#define ll long long\r\n#define N 130010\r\n#define PI acos(-1)\r\ninline int rd()\r\n{\r\n\tint x=0,f=1;char ch=getchar();\r\n\twhile(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}\r\n\twhile(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}\r\n\treturn x*f;\r\n}\r\nstruct cp{\r\n double r,i;\r\n cp(double _r=0,double _i=0):r(_r),i(_i){}\r\n cp operator + (cp x){return cp(r+x.r,i+x.i);}\r\n cp operator - (cp x){return cp(r-x.r,i-x.i);}\r\n cp operator * (cp x){return cp(r*x.r-i*x.i,r*x.i+i*x.r);}\r\n void clr(){r=i=0.0;}\r\n};\r\nint a[N];\r\nll ans;\r\nint r[N],l=-1;\r\ncp A[N],B[N];\r\nvoid FFT(cp *x,int n,int f)\r\n{\r\n\tint i,j,k;\r\n\tfor(i=0;i<n;i++) if(i<r[i]) swap(x[i],x[r[i]]);\r\n\tfor(i=1;i<n;i<<=1)\r\n\t{\r\n\t\tcp wn(cos(PI\/i),f*sin(PI\/i));\r\n\t\tfor(j=0;j<n;j+=(i<<1))\r\n\t\t{\r\n\t\t\tcp X,Y,w(1,0);\r\n\t\t\tfor(k=0;k<i;k++,w=w*wn)\r\n\t\t\t{\r\n\t\t\t\tX=x[k+j];Y=w*x[k+j+i];\r\n\t\t\t\tx[k+j]=X+Y;x[k+j+i]=X-Y;\r\n\t\t\t}\r\n\t\t}\r\n\t}\r\n\tif(f==-1) for(i=0;i<n;i++) x[i].r\/=n;\r\n}\r\nint ji[N];\r\nint n,nn,tot,bk=2000,L[N],R[N];\r\nvoid sol(int p)\r\n{\r\n\tfor(int i=0;i<nn;i++) A[i].clr(),B[i].clr();\r\n\tfor(int i=1;i<L[p];i++) A[a[i]].r+=1;\r\n\tfor(int i=R[p]+1;i<=n;i++) B[a[i]].r+=1;\r\n\tFFT(A,nn,1);FFT(B,nn,1);\r\n\tfor(int i=0;i<nn;i++) A[i]=A[i]*B[i];\r\n\tFFT(A,nn,-1);\r\n\tfor(int i=L[p];i<=R[p];i++) ans+=(int)(A[a[i]<<1].r+0.1);\r\n}\r\nint main()\r\n{\r\n\tn=rd();\r\n\tfor(int i=1;i<=n;i++) a[i]=rd();\r\n\tfor(nn=1;nn<=60000;nn<<=1)l++;\r\n\tfor(int i=0;i<nn;i++) r[i]=(r[i>>1]>>1)|((i&1)<<l);\r\n\twhile(tot*bk<n) L[++tot]=R[tot-1]+1,R[tot]=tot*bk;\r\n\tR[tot]=min(R[tot],n);\r\n\t\/\/ i,j,k in block\r\n\tfor(int p=1;p<=tot;p++)\r\n\t{\r\n\t\tfor(int i=L[p],k;i<R[p];i++)\r\n\t\t{\r\n\t\t\tfor(k=i+2;k<=R[p];k++)\r\n\t\t\t{\r\n\t\t\t\tji[a[k-1]]++;\r\n\t\t\t\tif(!((a[i]+a[k])&1))ans+=ji[(a[i]+a[k])>>1];\r\n\t\t\t}\r\n\t\t\tfor(k=i+1;k<R[p];k++) ji[a[k]]--;\r\n\t\t}\r\n\t}\r\n\t\/\/i,j in block\r\n\tmemset(ji,0,sizeof(ji));\r\n\tfor(int i=1;i<=n;i++) ji[a[i]]++;\r\n\tfor(int p=1;p<tot;p++)\r\n\t{\r\n\t\tfor(int i=L[p];i<=R[p];i++) ji[a[i]]--;\r\n\t\tfor(int i=L[p],j;i<R[p];i++)\r\n\t\t\tfor(j=i+1;j<=R[p];j++)\r\n\t\t\t\tif(a[j]*2-a[i]>=0)ans+=ji[a[j]*2-a[i]];\r\n\t}\r\n\t\/\/j,k in block\r\n\tmemset(ji,0,sizeof(ji));\r\n\tfor(int i=1;i<=n;i++) ji[a[i]]++;\r\n\tfor(int p=tot;p;p--)\r\n\t{\r\n\t\tfor(int i=L[p];i<=R[p];i++) ji[a[i]]--;\r\n\t\tfor(int j=L[p],k;j<R[p];j++)\r\n\t\t\tfor(k=j+1;k<=R[p];k++)\r\n\t\t\t\tif(a[j]*2-a[k]>=0) ans+=ji[a[j]*2-a[k]];\r\n\t}\r\n\t\/\/j in block FFT\r\n\tfor(int i=2;i<tot;i++) sol(i);\r\n\tprintf(\"%lld\\n\",ans);\r\n\treturn 0;\r\n}<\/pre>\n <\/p>\n","protected":false},"excerpt":{"rendered":"
3509: [CodeChef] COUNTARI Time Limit:\u00a040 Sec\u00a0\u00a0Me […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[38,37],"class_list":["post-396","post","type-post","status-publish","format-standard","hentry","category-bzoj","tag-fft","tag-37"],"_links":{"self":[{"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=\/wp\/v2\/posts\/396","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=396"}],"version-history":[{"count":2,"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=\/wp\/v2\/posts\/396\/revisions"}],"predecessor-version":[{"id":398,"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=\/wp\/v2\/posts\/396\/revisions\/398"}],"wp:attachment":[{"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=396"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=396"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.kylelv.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=396"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}