为什么这样改可以AC

现在的代码

#include<bits/stdc++.h>
using namespace std;
long long n,p,a[5000100],b[5000100],x,y,z,minn=500000001;
int main(){
    cin>>n>>p;
    for(long long i=0;i<n;i++){
        cin>>a[i];
    }
    for(long long i=0;i<p;i++){
        cin>>x>>y>>z;
        b[x-1]+=z;
        b[y]-=z;
    }
    for(long long i=0;i<n;i++){
        b[i]+=b[i-1];
        a[i]+=b[i];
        if(minn>a[i]){
            minn=a[i];
        }
    }
    cout<<minn;
    return 0;
}

之前的

#include<bits/stdc++.h>
using namespace std;
long long n,p,a[5000100],x,y,z,minn=500000001;
int main(){
    cin>>n>>p;
    for(long long i=0;i<n;i++){
        cin>>a[i];
    }
    for(long long i=0;i<p;i++){
        cin>>x>>y>>z;
        a[x-1]+=z;
        a[y]-=z;
    }
    for(long long i=0;i<n;i++){
        a[i]+=b[i-1];
        if(minn>a[i]){
            minn=a[i];
        }
    }
    cout<<minn;
    return 0;
}