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I want to use a minimum segment tree to answer no of elements which are less than k in a specific array range l to r in an unsorted array of integers. In the recursive function query I return the range of given tree element if the tree element lies within the range and also the minimum of this range is greater than k i.e. all the elements are greater than k, and return 0 if tree element points to only one array element and that too is less than k. Please tell me whats wrong with my approach.

Here we take no of elements and no of queries as input n and q, then for next n lines we take the array elements as input. Then for each query we need to answer the no of elements in the range [l,r] that are greater than k.

void buildTree(int *a,int s,int e,int *tree,int index){
    //Base Case
    if(s==e){
        tree[index]=a[s];
        return;
    }
    //Recursive case
    int mid=(s+e)/2;
    buildTree(a,s,mid,tree,2*index);
    buildTree(a,mid+1,e,tree,2*index+1);
    tree[index]=min(tree[2*index],tree[2*index+1]);
    return;
}
int query(int *tree,int ss,int se,int qs,int qe,int k,int index){
    //Complete Overlap
    if(se==ss&&tree[index]<k)return 0;
    if(ss>=qs && se<=qe&& tree[index]>=k)
     return se-ss+1;
     //No Overlap
    if(ss>qe || qs>se)
     return 0;
     //Partial Overlap
    int mid=(ss+se)/2;
    int leftAns=query(tree,ss,mid,qs,qe,k,2*index);
    int rightAns=query(tree,mid+1,se,qs,qe,k,2*index+1);
    return leftAns+rightAns;
}
void updateNode(int *tree,int ss,int se,int i,int update,int index){
    //No Overlap
    if(i>se||i<ss)
    return;
    //LeafNode
    if(ss==se){
        tree[index]=update;
        return;
    }
    //Partial Overlap
    int mid=(ss+se)/2;
    updateNode(tree,ss,mid,i,update,2*index);
    updateNode(tree,mid+1,se,i,update,2*index+1);
    tree[index]=min(tree[2*index],tree[2*index+1]);
    return;
   }

int main(){
int n,q;
cin>>n>>q;
int arr[n];
for(int i=0;i<n;i++)cin>>arr[i];
int tree[4*n]; 
buildTree(a,0,n-1,tree,1);
for(int i=0;i<q;i++){
int l,r,k;
cin>>l>>r>>k;
cout<<query(tree,0,n-1,l-1,r-1,k,1)<<"\n";
}
return 0;
}
Tanmay Shukla
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  • I do not see any tree here. Is this sorted arrray? – jenkas Jan 06 '20 at 20:36
  • 1
    [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). Second, you wrote all of this code, and didn't know whether it would solve the problem or not? Did you plan this on paper first, before writing any code? Once the program is written, you should easily see where the code you wrote diverges from the original plan, and either fix the code to follow your plan, or realize that the plan is flawed and you need to start over. – PaulMcKenzie Jan 06 '20 at 21:44
  • I have edited the post, here we take no of elements and no of queries as input n and q, then for next n lines we take the array elements as input. Then for each query we need to answer the no of elements in the range [l,r] that are greater than k. – Tanmay Shukla Jan 08 '20 at 00:13

0 Answers0