TraceCode

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Master algorithms by seeing them work. Write solutions to coding problems and trace every variable, pointer, and state change live from your code.

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Java

1import java.util.HashMap;

2import java.util.Map;

4class Solution {

5 public int lengthOfLongestSubstring(String s) {

6 Map<Character, Integer> lastSeen = new HashMap<>();

7 int left = 0;

8 int best = 0;

10 for (int right = 0; right < s.length(); right++) {

11 char ch = s.charAt(right);

12 if (lastSeen.containsKey(ch) && lastSeen.get(ch) >= left) {

13 left = lastSeen.get(ch) + 1;

14 }

15 lastSeen.put(ch, right);

16 int len = right - left + 1;

17 if (len > best) {

18 best = len;

19 }

20 }

22 return best;

23 }

24}

1/1

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You learn a pattern, practice it on a real problem, then test yourself in a timed interview.

01Learn the pattern

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Sliding Window

Arrays & Strings

Scan contiguous subarrays/substrings by moving a window and updating your answer in O(1) per move.

02Figure it out yourself

Predict each step before you see it

Each problem becomes a series of small questions. You answer each one before the engine shows the next step.

Try a problem

Predict

How many character reads does this do on a long unique string?

s is 1000 distinct characters, so no walk ever stops early.

03Prove it under pressure

Take a timed mock interview

Timed assessments built on realistic, company-style problems. Your code runs against hidden tests, and you get feedback after each session.

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