Question

The following is the implementation of http://www.spoj.pl/problems/LITE/ using Segment Tree’s with lazy propagation. I am new to segment trees and I cannot understand why I am getting TLE. Could someone please look at it and help me correct my error?

#include <iostream>
#include <iostream>
#include <cstdio>
#include <cstring>
#define MAX 100000
using namespace std;
int M[2*MAX+1];
int flag[2*MAX+1];
int count;
void refresh(int begin,int end,int n)
{
    M[n] = end-begin+1 - M[n];
    flag[n]=0;
    flag[n*2] =!flag[n*2];
    flag[n*2+1] =!flag[n*2+1];
}
void update(int begin,int end,int i,int j,int n=1)
{
    if(flag[n])
    {
        refresh(begin,end,n);
    }
    if(begin>=i && end<=j)
    {
        if(!flag[n])
        {
            refresh(begin,end,n);
        }
        flag[n] = 0;
        return;
    }
    else if(begin>=end)
    {
        return;
    }
    else
    {
        int mid = (begin+end)>>1;
        if(i<=mid)
        {
            update(begin,mid,i,j,n*2);
        }
        if(j>mid)
        {
            update(mid+1,end,i,j,n*2+1);
        }
        if(flag[2*n])
        {
            refresh(begin,mid,2*n);
        }
        if(flag[2*n+1])
        {
            refresh(mid+1,end,2*n+1);
        }
        M[n] = M[n*2]+ M[n*2+1];
    }
}
int query(int begin,int end,int i,int j,int n=1)
{
    if(flag[n])
    {
        refresh(begin,end,n);
    }
    if(begin>=i && end<=j)
    {
        return M[n];
    }
    if(begin>=end)
    {
        return 0;
    }
    int mid = (begin+end)>>1;
    int l=0,r=0;
    if(i<=mid)
    {
        l = query(begin,mid,i,j,n*2);
    }
    if(j>mid)
    {
        r = query(mid+1,end,i,j,n*2+1);
    }
    if(flag[2*n])
    {
        refresh(begin,mid,2*n);
    }
    if(flag[2*n+1])
    {
        refresh(mid+1,end,2*n+1);
    }
    M[n] = M[n*2]+ M[n*2+1];
    return l+r;
}
int main()
{
    memset(M,0,sizeof M);
    int n,m,a,b,c;
    scanf("%d%d",&n,&m);
    for(int i=0; i<m; i++)
    {
        scanf("%d%d%d",&a,&b,&c);
        if(a==0)
        {
            update(1,n,b,c);
        }
        else
        {
            printf("%d\n",query(1,n,b,c));
        }
    }
    return 0;
}

Answer 1

M[node]^=1; might be faster than M[node] = (M[node]==0)?1:0;, and (begin+end)>>1 faster than (begin/end)/2, but not very relevant

LE: Try if making the recursive functions inline will run faster. I think it unravels the recursion a couple of times and works a little bit faster. Maybe sending the parameters as references will make it run faster, try that out. If the test cases are chosen properly you still shouldn’t be able to pass the tests with this trickery, but it helps sometimes.