前缀和

前缀和

题目：https://www.acwing.com/activity/content/problem/content/829/1/

#include<iostream>
using namespace std;
const int maxn = 1e5;
int n, m, l, r;
int a[maxn], num[maxn];
int main()
{
    cin >> n >> m;
    for(int i = 1; i <= n; i++)
        cin >> a[i];
    for(int i = 1; i <= n; i++)
        a[i] = a[i] + a[i - 1];
    while(m--)
    {
        cin >> l >> r;
        cout << a[r] - a[l - 1] << " " << endl;
    }
    return 0;
}

 

 

子矩阵的前缀和

 

#include<iostream>
using namespace std;
const int maxn = 1000+10;
int n, m, q, x1, y1, x2, y2;
int s[maxn][maxn], a[maxn][maxn];
int main()
{
    cin >> n >> m >> q;
    for(int i = 1; i <= n; i ++)
        for(int j = 1; j <= m; j ++)
            cin >> a[i][j];
    for(int i = 1; i <= n; i ++)
        for(int j = 1; j <= m; j ++)
            s[i][j] = s[i-1][j] + s[i][j-1] - s[i-1][j-1] + a[i][j];
    while(q--)
    {
        cin >> x1 >> y1 >> x2 >> y2;
        cout << s[x2][y2] - s[x2][y1-1] - s[x1-1][y2] + s[x1-1][y1-1] << endl;
    }
    return 0;
}