Codeforces Round #197 (Div. 2), problem: (D) Xenia and Bit Operations

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

#include<bits/stdc++.h>

using namespace std;
#define pb push_back
#define pii pair<int,int>
#define pli pair<long long int,int>
#define pbi pair<bool,int>
#define pob pop_back
#define all(a) a.begin(),a.end()
#define fio ios_base::sync_with_stdio(false); cin.tie(0);cout.tie(0);
#define i64 long long int
#define mem(x,y) memset(x,y,sizeof(x))
#define fill(arr,b) fill(arr, arr+sizeof(arr)/sizeof(arr[0]), b)
#define maxarr(n,arr,m) m =-1e9;for(int i=0;i<n;i++)if(arr[i]>m) m=arr[i];
#define read freopen("input.txt","r",stdin);
#define F first
#define S second

const int maxn = (i64)1e9+7;
const double pi = acos(-1.0);

//bool fne(const pii &a, const pii &b){return (a.F < b.F)||(a.F==b.F&&a.S<b.S);}
//bool sne(const pii &a, const pii &b){return (a.S < b.S)||(a.S==b.S&&a.F<b.F);}

int arr[140005];
int tree[560005];

void init(int node, int b, int e,bool fg){
    if(b == e){
         tree[node] = arr[b];
         return;
    }
    int left = node<<1;
    int right = (node<<1) + 1;
    int m = (b+e)>>1;
    init(left,b,m,!fg);
    init(right,m+1,e,!fg);
    if(fg) tree[node] = tree[left]|tree[right];
    else   tree[node] = tree[left]^tree[right];
}
void update(int node, int b, int e, int i, int value,bool fg){
    if(i<b || i>e) return;
    else if(b>=i && e<=i){
        tree[node] = value;
        return;
    }
    int left = node<<1;
    int right = (node<<1) + 1;
    int m = (b+e)/2;
    update(left,b,m,i,value,!fg);
    update(right,m+1,e,i,value,!fg);
    if(fg) tree[node] = tree[left]|tree[right];
    else   tree[node] = tree[left]^tree[right];
}
int main()
{
    //read;
    int n,m;
    scanf("%d%d",&n,&m);
    bool fg=n%2;
    n=1<<n;
    for(int i=1;i<=n;i++) scanf("%d",&arr[i]);
    if(!fg) init(1,1,n,0);
    else init(1,1,n,1);
    //printf("%d\n",tree[1]);
    while(m--)
    {
        int p,q;
        scanf("%d%d",&p,&q);
        if(!fg) update(1,1,n,p,q,0);
        else update(1,1,n,p,q,1);
        printf("%d\n",tree[1]);
    }

    return 0;
}