Query the primary line segment tree in the update segment of HDOJ 1698

In the blog of the NOS gods
This is usually a hurdle for beginners.
Scared me not to look at the line tree for three days = because I am a newbie .....
Before going to bed last night, I want to understand the updated line segment tree of the segment query section. The result is also true;
This line segment tree uses the delay mark and Its Operation PushDown;
The latency mark indicates the value of the current interval.

PushDown is used to push down the current point if there is a delay mark, and the rear is zero.

#include<iostream>#define MAXN 100001using namespace std;int sum[ MAXN<<2 ];int sign[ MAXN<<2 ];void PushUp( int rt ){     sum[rt]=sum[rt<<1]+sum[rt<<1|1];}void PushDown( int rt,int m ){     if( sign[rt] )     {         sign[rt<<1]=sign[rt<<1|1]=sign[rt];         sum[rt<<1]=(m-(m>>1))*sign[rt];         sum[rt<<1|1]=(m>>1)*sign[rt];         sign[rt]=0;     }     return ;}void build( int l,int r,int rt ){      sign[rt]=0;      sum[rt]=1;      if( l==r )return ;     int m=(l+r)>>1;     build( l,m,rt<<1 );     build( m+1,r,rt<<1|1 );     PushUp(rt);     return ;}void update( int l,int r,int L,int R,int value,int root ){     if( l<=L&&R<=r )     {         sign[root]=value;         sum[root]=value*(R-L+1);         return ;     }     PushDown( root,R-L+1 );     int m=( L+R )>>1;     if( l<=m )update( l,r,L,m,value,root<<1 );     if( r>m )update( l,r,m+1,R,value,root<<1|1 );     PushUp(root);     return ;}int main(){    int loop;    int cas=1;    scanf( "%d",&loop );    while( loop-- )    {           int size,n,i;           scanf( "%d %d",&size,&n );            build( 1,size,1 );           while( n-- )           {                  int a,b,c;                  scanf( "%d %d %d",&a,&b,&c );                  update( a,b,1,size,c,1);           }           printf("Case %d: The total value of the hook is %d.\n",cas++ , sum[1]);    }    return 0; }