Leetcode #394: Decode String
In this guide, we solve Leetcode #394 Decode String in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given an encoded string, return its decoded string. The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Stack, Recursion, String
Intuition
The problem has a natural nested or last-in-first-out structure.
A stack lets us resolve matches in the correct order as we scan.
Approach
Push items as they appear and pop when you can finalize a decision.
The stack captures the unresolved part of the input.
Steps:
- Push elements as you scan.
- Pop when a rule or match is satisfied.
- Use the stack to compute results.
Example
Input: s = "3[a]2[bc]"
Output: "aaabcbc"
Python Solution
class Solution:
def decodeString(self, s: str) -> str:
s1, s2 = [], []
num, res = 0, ''
for c in s:
if c.isdigit():
num = num * 10 + int(c)
elif c == '[':
s1.append(num)
s2.append(res)
num, res = 0, ''
elif c == ']':
res = s2.pop() + res * s1.pop()
else:
res += c
return res
Complexity
The time complexity is O(n). The space complexity is O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.