Build a prefix array — practice exercise

What you'll be asked to do

1. 1.Build a prefix arrayexercise · easy · 10 min

   Build running totals with a leading zero and join them.

   Passes when: #out should show 0-3-4-8.

2. 2.Sum a rangeexercise · easy · 10 min · practice plan

   Use prefix sums to total indices two through five.

   Passes when: #out should show sum 19.

3. 3.Whole array totalexercise · easy · 10 min · practice plan

   Show the total of every value using the prefix array.

   Passes when: #out should show sum 31.

4. 4.Count subarrays with a sumexercise · easy · 10 min · practice plan

   Count runs adding to seven using a hash map.

   Passes when: #out should show found 4.

5. 5.Updates need a treeexercise · medium · 10 min · practice plan

   Report the structure needed when values change between queries.

   Passes when: #out should show fenwick tree.

6. 6.Fix the prefix without a leading zerobug fix · easy · 8 min

   Starting at the first value makes every range need a special case.

   Passes when: #out should show 0-3-4-8.

7. 7.Fix the range bounds off by onebug fix · easy · 8 min · practice plan

   The range sum subtracts the prefix before the start, not at it.

   Passes when: #out should show sum 19.

8. 8.Fix the counter missing the empty prefixbug fix · medium · 8 min · practice plan

   Without seeding zero, runs starting at index zero are never counted.

   Passes when: #out should show found 4.

9. 9.Fix the running total recorded firstbug fix · easy · 8 min · practice plan

   Storing before looking up lets a run match itself when the target is zero.

   Passes when: #out should show found 0.

10. 10.Fix the prefix array used with updatesbug fix · easy · 8 min · practice plan

    One change invalidates every later entry, so a tree is needed.

    Passes when: #out should show fenwick tree.

Between them these tasks cover Prefix arrays, Range sums and Fenwick trees.

Looking for a different chapter? See every practice set, or switch to the interview theory questions.