树状数组入门

#include<stdio.h>
const int M = 1e5 ;
int a[M] , c[M];
int n , m ;

void modify (int x , int add )
{
    for (; x <= n ; x += x & (-x))
        c[x] += add ;
}

int sum_get (int x)
{
    int ret = 0 ;
    for (; x > 0 ; x -= x & (-x))
        ret += c[x] ;
    return ret ;
}
/*
void modify (int x , int y , int add)//二维
{
    for (; x <= n ; x += x & (-x)) {
        for (; x <= n ; y += y & (-y)) {
            c[x][y] += add ;
        }
    }
}

int get_sum (int x , int y )
{
    int ret = 0 ;
    for (; x > 0 ; x -= x & (-x)) {
        for (; y > 0 ; y -= y & (-y)){
            ret += c[x][y] ;
        }
    }
    return ret ;
}
*/
int main ()
{
    freopen ("a.txt" , "r" , stdin ) ;
    scanf ("%d%d" , &n , &m) ;
    for (int i = 1 ; i <= n ; i ++) {
        scanf ("%d" , &a[i]) ;
        modify (i , a[i]) ;
    }
    while (m --) {
        int x ;
        scanf ("%d" , &x) ;
        printf ("%d\n" , sum_get (x)) ;
    }
    return 0 ;
}