关于树状数组初次接触者的疑问

struct tree{
	int num;
	int lid,rid;
	int left,right;
};
tree leaf[1000005];int cnt=0;
int a[100005];
inline int build(int l,int r){
	leaf[++cnt]={0,0,0,l,r};
	if(l==r){leaf[cnt].num=a[l];return cnt;}
	int mid=l+r>>1;
	leaf[cnt].lid=build(l,mid);
	leaf[cnt].rid=build(mid+1,r);
	leaf[cnt].num=leaf[leaf[cnt].rid].num+leaf[leaf[cnt].lid].num;
	return cnt;
}