区间和

文章讲解：https://www.programmercarl.com/kamacoder/0058.%E5%8C%BA%E9%97%B4%E5%92%8C.html

1.前缀和，ACM模式。

CPP

#include<iostream>
#include<vector>
using namespace std;

int main(){
    int n;
    int a,b;
    cin>>n;
    vector<int> arr(n,0);
    vector<int> res(n,0);
    
    for(int i = 0;i <n;i++){
        cin>>arr[i];
    }
    int sum =0;
    for(int i = 0;i<n;i++){
        sum += arr[i];
        res[i] = sum;
    }
    int result ;
    while(cin>>a>>b){
        
        if(a == 0)
            result =  res[b];
        result =  res[b] - res[a-1];  
        cout<<result<<endl;  
        
    }
    
    
    
}