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A-008● Day 082026.05.10mediumleetcode #271neetcode150

Encode and Decode Strings

#array#string#design
↗ leetcode #271 · encode-and-decode-strings
01

Problem

· problem
P.008

Encode and Decode Strings

↗ leetcode #271

Design an encoder and decoder for a list of strings. The encode function converts a list of strings into a single encoded string. The decode function takes the encoded string and reconstructs the original list of strings. The encoding scheme must reliably handle arbitrary strings, including those containing special characters and empty strings.

constraints
  • · 1 ≤ number of strings ≤ 1e2
  • · 0 ≤ length of each string ≤ 1e4
  • · Strings can contain any ASCII characters
// paraphrased summary — see source for full text
examples
example 1input → output
["Hello","World"]
5#Hello5#World
example 2input → output
[""]
0#
02

Pre-solve

· pre-solve
● 1list shown→● 2select→● 3reveal
  • ☐
    How do we distinguish where one encoded string ends and another begins?
  • ☐
    How are empty strings handled in the encoding?
  • ☐
    What if the input strings contain the '#' delimiter character?
  • ☐
    Does the solution efficiently handle very large strings?
  • ☐
    Wouldn't using CSV format with commas be simpler?
  • ☐
    Wouldn't fixed-width length fields be better than variable-length numbers?
  • ☐
    How does the decoder identify where the length number ends?
check the items you would ask, then press confirm
// session-only state — refresh resets (repeatable practice)
03

Logic Structure

· logic
● 1slots shown→● 2pick per slot→● 3reveal
// pick one code line per slot to assemble the algorithm flow. no typing — just the logic skeleton.
step 1· Initialize result string
○
res = ""
○
res = []
○
res = ""s
step 2· Iterate through each string
○
for s in strs:
○
for i in range(len(strs)):
○
while strs:
step 3· Append length marker and string│ nested
○
res += str(len(s)) + "#" + s
○
res += s + "#" + str(len(s))
○
res += "#" + str(len(s)) + s
○
res += str(len(s)) + s
step 4· Return the encoded string
○
return res
○
return res[:-1]
○
return ""
pick one option per slot
// format: slot — recursive / DP patterns use ordering / state-first formats. ADR-08 follow-up.
04

Solve · Trace

· solve
solution.py
1
class Solution:
2
    def encode(self, strs):
3
        res = ""
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        for s in strs:
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            res += str(len(s)) + "#" + s
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        return res
7
8
    def decode(self, s):
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        res = []
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        i = 0
11
        
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        while i < len(s):
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            j = i
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            while s[j] != '#':
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                j += 1
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            length = int(s[i:j])
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            i = j + 1
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            j = i + length
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            res.append(s[i:j])
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            i = j
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        return res
mental dry-run cases
// walk each case in your head; expand the worked example below if stuck.
case 1
["Hello","World"]
→
5#Hello5#World
case 2
[""]
→
0#
// UI does not walk-through — you do the dry-run mentally. Expand the worked example if stuck.