Coding interviews
using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
string[] input={"b:2","a:1","b:3","c:0"};
var totals=new Dictionary<string,long>(StringComparer.Ordinal);
foreach(string line in input) { int split=line.IndexOf(':'); if(split<=0 || line.IndexOf(':',split+1)>=0 || !int.TryParse(line.AsSpan(split+1),NumberStyles.None,CultureInfo.InvariantCulture,out int value)) throw new FormatException("record"); string key=line[..split]; totals[key]=checked(totals.GetValueOrDefault(key)+value); }
var ranked=totals.OrderByDescending(x=>x.Value).ThenBy(x=>x.Key,StringComparer.Ordinal).ToArray();
Console.WriteLine("ranked:"+string.Join(",",ranked.Select(x=>x.Key+"="+x.Value)));
var page=ranked.Skip(1).Take(2); Console.WriteLine("page:"+string.Join(",",page.Select(x=>x.Key)));
Console.WriteLine("input retained:"+input.Length);
Reveal answer and reasoning
The output lines, in order, are “ranked:b=5,a=1,c=0”; “page:a,c”; “input retained:4”. b receives two and three for five, a receives one and c retains zero. Ranking produces b=5,a=1,c=0. Skipping one and taking two yields a,c, and the original four input strings remain available. Parsing rejects malformed delimiters or nonconforming numeric text instead of silently dropping a bad record. The first colon is located once and a second colon is rejected.
TryParse uses NumberStyles.None with invariant culture, so signs and surrounding whitespace are not silently admitted as quantities. The rank snapshot is created before paging, which makes the displayed offset refer to one known ordered result rather than a changing source.
For n input code units and u distinct keys, parsing and hashing examine text, aggregation uses expected dictionary work and ranking adds O(u log u) comparisons with O(u) retained totals and ranked storage. A two-item page bounds only output, not all work or memory. Combining correct helpers can still produce a wrong system when their contracts disagree. The numeric grammar disallows signs and whitespace, whereas keys are preserved exactly.
The fixed snapshot makes offset paging stable for this run; it does not make offsets safe across independent requests over changing data.
What a strong answer covers
- Exact output: ranked:b=5,a=1,c=0 | page:a,c | input retained:4
- b receives two and three for five, a receives one and c retains zero.
- Combining correct helpers can still produce a wrong system when their contracts disagree.
Common pitfall
Combining correct helpers can still produce a wrong system when their contracts disagree. The numeric grammar disallows signs and whitespace, whereas keys are preserved exactly. The fixed snapshot makes offset paging stable for this run; it does not make offsets safe across independent requests over changing data.
Follow-up questions
- Add malformed quantities, duplicate delimiters, tied totals and zero-valued rows. Hint: test failures separately from the successful fixture so one bad row does not obscure ordering checks. Verify that input remains unchanged.
- How would returning all parse errors change the publication and memory contract?
Self-review
Choose what to revisit. These marks are your own assessment and are separate from lesson completion.