思路：

1. 求前缀和：preSum[i]=preSum[i-1]+arr[i],arr的下标从1开始
2. 用前缀和[l,r]：preSum[r]-preSum[l-1]

c++

用时 150ms 左右

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
const int N=100010;
int preSum[N];
int n,m;
int main(){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++){
        int x;
        scanf("%d",&x);
        preSum[i]=preSum[i-1]+x;
    }
    while(m--){
        int l,r;
        scanf("%d%d",&l,&r);
        printf("%d\n",preSum[r]-preSum[l-1]);
    }
    return 0;
}

java

用时 1835ms 左右

import java.util.*;
import java.io.*;

public class Main{
    public static void main(String[] args) throws Exception{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw=new PrintWriter(new OutputStreamWriter(System.out));
        String[] line1=br.readLine().split(" ");
        int n=Integer.parseInt(line1[0]),m=Integer.parseInt(line1[1]);
        String[] line2=br.readLine().split(" ");
        //求前缀和
        int[] preSum=new int[n+10];
        for(int i=1;i<=n;i++) preSum[i]=preSum[i-1]+Integer.parseInt(line2[i-1]);
        //用前缀和
        while(m>0){
            m--;
            String[] line3=br.readLine().split(" ");
            int l=Integer.parseInt(line3[0]),r=Integer.parseInt(line3[1]);
            pw.println(preSum[r]-preSum[l-1]);
        }
        pw.flush();
    }
}

python

用时 2670 ms左右

n,m=map(int,input().split())
line=input()
lst=[int(i) for i in line.split()]

#求前缀和
preSum=[0,]
for i in range(1,n+1):
    preSum.append(preSum[i-1]+lst[i-1])

#用前缀和
for i in range(m):
    l,r=map(int,input().split())
    print(preSum[r]-preSum[l-1])