Longest stretch with no repeat
A token generator checks its output for the longest run of characters in which nothing appears twice.
- The run has to be a single unbroken stretch, not a selection.
- Every character counts, including spaces and punctuation.
- Empty text has a longest run of zero.
LongestUniqueRun(text: string) → int
C# needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
public int LongestUniqueRun(string text) {
}Worked examples
| Call | Result |
|---|---|
LongestUniqueRun("abcabcbb") | 3 |
LongestUniqueRun("bbbbb") | 1 |
LongestUniqueRun("pwwkew") | 3 |
LongestUniqueRun("abcdef") | 6 |
Hint
Slide a window. When a repeat comes in, pull the left edge past where that character was last seen — never backwards.
Reference solution in C#
public int LongestUniqueRun(string text) {
var lastAt = new Dictionary<char, int>();
int best = 0, start = 0;
for (int i = 0; i < text.Length; i++) {
char c = text[i];
if (lastAt.ContainsKey(c) && lastAt[c] >= start) start = lastAt[c] + 1;
lastAt[c] = i;
best = Math.Max(best, i - start + 1);
}
return best;
}