Best time to buy and sell
StopSliding window
- CH1 buy
- CH2 sell
8/8
The window runs from buy day to sell day. Buy moves to any cheaper day.
Step through it →Sliding Window
A sliding window grows on the right and shrinks on the left, so each element enters and leaves once instead of being scanned again.
Longest substring without a repeated character
StopSliding window
s
- CH1 left
- CH2 right
left = 0, best = 0, and last_index is empty.
Typical cost O(n) time, O(1)–O(k) spaceFull lesson: Longest Substring Without Repeating Characters →
When to reach for it
- The question is about a contiguous run: a substring or a subarray.
- You want the longest, shortest or best run that follows a rule.
- Adding or dropping one element updates the rule cheaply: a count, a sum, a last-seen index.
- The brute force rescans ranges that overlap.
What stays true
Right adds one character. If that makes a repeat, left jumps past the earlier copy before anything is measured. So every length the stage reads is a valid window's.
The window is always valid
StopSliding window
InvariantThe window never holds a repeated character.✓ Holds
- CH1 left
- CH2 right
left = 0, best = 0, and last_index is empty.
Break it
Each input here has tripped up a version of this code. The stage runs the one that gets it right.
The last 'a' was seen at 0, before left. Jumping left to 1 would move it backwards and let 'bb' back in. It jumps only for a sighting inside the window.
Longest substring on the inputs that trip it up
StopSliding window
- CH1 left
- CH2 right
left = 0, best = 0, and last_index is empty.
Answer2Steps11
Variants
Character replacement
StopSliding window
- CH1 left
- CH2 right
12/12
The window never shrinks. It slides when more than k letters would need changing.
Step through it →Minimum window substring
StopSliding window
- CH1 left
- CH2 right
18/18
Grow until the window covers t, then shrink while it still does.
Step through it →The code
The same algorithm in plain Python. The lit line is the one the current frame came from.
Longest substring without a repeated character
StopSliding window
- CH1 left
- CH2 right
left = 0, best = 0, and last_index is empty.
longest_unique_substring.py
def length_of_longest_substring(s): left = 0 best = 0 last_index = {} for right in range(len(s)): c = s[right] if c in last_index and last_index[c] >= left: left = last_index[c] + 1 last_index[c] = right best = max(best, right - left + 1) return bestDrill it
Best Time to Buy and Sell StockRuns live,EasyLongest Substring Without Repeating CharactersRuns live,MediumLongest Repeating Character ReplacementRuns live,MediumMinimum Window SubstringRuns live,Hard
Minimum Size Subarray SumMedium
Permutation in StringMedium