codeforce617E-XOR and Favorite Number Mo team + XOR prefix sum

Portal: http://codeforces.com/contest/617/problem/E

The meaning of the question:
Given a series of numbers, for each query interval, calculate how many sub-intervals are XORed as k.
Idea:
You can preprocess the XOR prefix first, the XOR value of an interval [L, R] = sum[R]^sum[L-1];
if the current interval is [a, b], add a right endpoint b+ 1, then the contribution of this b+1 is how many sum[x]=sum[b+1]^k in the [a,b] interval,
then we can record num[sum[x]] each time, that is, num[sum [b+1]^k], and record num[sum[b+1]]++, the same goes for the left interval.

Then we can use the Mo team algorithm.

#include <iostream>
#include <cstdio>
#include <algorithm>
typedef long long ll;
using namespace std;
const int maxn = 1<<20;
ll a[maxn];
ll sum = 0, ans[maxn];
int k,B = 233,cnt[maxn];
#define bel(x) ((x-1)/B +1)
struct node{
    int id,l,r;
}q[maxn];

bool cmp (node a, node b)
{
    if(bel(a.l) == bel(b.l))
        return a.r < b.r;
    return bel(a.l) < bel(b.l);
}
void add(int x)
{
    sum += cnt[x^ k];
    cnt[x] ++;
}
void del (int x)
{
    cnt[x] --;
    sum = cnt[x^ k];
}
int main(){
    int n,m;
    scanf("%d%d%d",&n,&m,&k);
    for(int i=1; i<=n; i++)
    {
        scanf("%I64d",&a[i]);
        a[i] ^= a[i-1];
    }
    for(int i = 1; i <= m; i++)
    {
        scanf("%d%d",&q[i].l,&q[i].r);
        q[i].id = i;
    }
    sort(q+1, q+m+1, cmp);
    ll ql = 1,qr = 0;
    cnt[0] = 1;
    for(int i=1; i<=m; i++)
    {
        while(ql < q[i].l) del(a[ql-1]),ql++;
        while(ql > q[i].l) ql--,add(a[ql-1]);
        while(qr > q[i].r) del (a[qr--]);
        while(qr < q[i].r) add (a[++qr]);
        ans[q[i].id] = sum; 
    }
    for(int i=1; i<=m; i++)
        printf("%I64d\n",ans[i]);
    return 0;
}