Leetcode 394. Decode String
Decode a string containing nested repeat patterns of the form k[encoded_string], expanding each bracketed segment k times (k may be multi-digit) possibly with multiple nesting levels. Core challenge: parse and evaluate the nested structure correctly and efficiently (e.g., via stack or recursion) to produce the fully decoded string.
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Microsoft
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Early June, 2026
Target
Mid-level
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. Note that k is guaranteed to be a positive integer. You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there will not be input like 3a or 2[4]. The test cases are generated so that the length of the output will never exceed 105. Example 1: Input: s = "3[a]2[bc]" Output: "aaabcbc" Example 2: Input: s = "3[a2[c]]" Output: "accaccacc" Example 3: Input: s = "2[abc]3[cd]ef" Output: "abcabccdcdcdef"
Late October, 2025
Microsoft
Senior
Late December, 2024
Mid-level
Decode string
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