36 lines
698 B
Java
36 lines
698 B
Java
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package LanQiaoOJ;
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import java.util.Scanner;
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/*
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* <EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>n<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>a1,a2,<EFBFBD><EFBFBD><EFBFBD><EFBFBD>,am,<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
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<EFBFBD><EFBFBD><EFBFBD>ӵĺͣ<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
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S=a1<EFBFBD><EFBFBD>a2+a1<EFBFBD><EFBFBD>a3+<EFBFBD><EFBFBD>+a1<EFBFBD><EFBFBD>am+a2<EFBFBD><EFBFBD>a3+
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ʽ
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ĵ<EFBFBD>һ<EFBFBD>а<EFBFBD><EFBFBD><EFBFBD>һ<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
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<EFBFBD>ڶ<EFBFBD><EFBFBD>а<EFBFBD><EFBFBD><EFBFBD>n<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>a1,a2,<EFBFBD><EFBFBD><EFBFBD><EFBFBD>,an*/
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//<2F><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ϲ<EFBFBD><CFB2><EFBFBD><EFBFBD>⣺S1=a1(a2+a3+...+an-1)
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public class <EFBFBD><EFBFBD><EFBFBD><EFBFBD> {
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static int N=200010;
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static long[]a=new long[N];
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public static void main(String[] args) {
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Scanner sc=new Scanner(System.in);
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int n=sc.nextInt();
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for(int i=1;i<=n;i++) {
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a[i]=sc.nextInt();
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}
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//<2F><><EFBFBD>ս<EFBFBD><D5BD><EFBFBD>
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long ans=0;
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//<2F><>¼i-1<><31><EFBFBD><EFBFBD><EFBFBD>ĺ<EFBFBD>
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long sum=0;
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for(int i=1;i<=n;i++) {
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ans+=a[i]*sum;
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sum+=a[i];
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}
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//ʱ<>临<EFBFBD>Ӷ<EFBFBD>ΪO(n)
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System.out.print(ans);
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}
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}
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