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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Mid July, 2026
Let's assume we have encoded strings and we want to expand them. The encoding rule is k[char], k{char}, k(char) - will repeat the character k times. This format is guaranteed. However, cannot guarantee parentheses are valid: k[char}, or k}char{ Examples: 3}a}2[b] invalid case 3[a] = aaa 3{a}2[b] = aaabb 3(2[a]b) = aabaabaab 2[a] aa; aab 3 times -> aab aab aab 2[b]cd = bbcd 3(aab) aabaabaab
Mid June, 2026
k[ecnoded_string] - k is positive integer, decode string. and string appears exactly k time in decoded string
Early June, 2026
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"
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