Segment Tree

int seg_tree[4*N];     // seg_tree[i] = value at node number i
void build(int s ,int e, int node_num) // [s..e] is the range of node:- node_num
{
    if(s==e) // We reach a leaf node
    {
      seg_tree[node_num] = A[s];
      return;
    }
    int m = (s+e)/2; // splitting the current range
    build(s,m,node_num*2); // building then left subtree
    build(m+1,e,node_num*2+1); // building the right subtree
 
    // Updating value at current node after building its left and right subtree
 
    seg_tree[node_num] = seg_tree[node_num*2] + seg_tree[node_num*2+1];
 
   return;
}

int query(int l,int r,int s,int e,int node_num)
{
    if(s>r || e=l && e<=r) // the current range is completely inside query range
      return seg_tree[node_num];   // returning the sum of current range
    else // the query range is inside the current range
    {
      int m = (s+e)/2;  // splitting the current range and querying its children
      return query(l,r,s,m,node_num*2) + query(l,r,m+1,e,node_num*2+1);
    }
}

void point_update(int p, int newval, int s, int e, int node_num)
{
   if(s==e)   // We reached the point of update
   {          // base case of recursion
     seg_tree[node_num] = newval;   // updating the point
     return;
    }
   int m = (s+e)/2;   // splitting the range 
   if(p<=m)   // The point lies in the left subtree [l..m]
     point_update(p,newval,s,m,node_num*2);
   else       // The pint lies in the right subtree [m+1..r]
     point_update(p,new_val,m+1,e,node_num*2+1);
 
   // Updating the current node after its children are updated
   seg_tree[node_num] = seg_tree[node_num*2] + seg_tree[node_num*2+1];
}