How to solve this one?

#1

Find the starting index of segment tree using this build function :

void build(int node, int start, int end) {

if(start == end)
{
tree[node] = A[start];
}
else
{
int mid = (start + end) / 2;
build(2node, start, mid);
build(2node+1, mid+1, end);
tree[node] = tree[2node] + tree[2node+1];
}
}

#2 
 tree[node] = tree[2*node] + tree[2*node+1];

this line will tell
the start point
see if we take start as 0
then 2node = 0
and 2 * node +1 == 1
so that would suffice the condition of nodes being at 2node &2*node+1

#3

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