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Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 736: Parse Lisp Expression

Problem Restatement We are given a string expression that represents a Lisp-like expression. We need to evaluate it and return its integer value. An expression can be one of these forms: Form Meaning Integer A positive or negative integer Variable A name whose value comes from a surrounding let add Adds two expressions mult Multiplies two expressions let Assigns variables, then evaluates a final expression The official syntax includes integer...
leetcode string recursion hash-table parsing May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 640: Solve the Equation

Problem Restatement We are given a linear equation as a string. The equation contains: Token Meaning x The variable integers Constant terms or coefficients + and - Operations = Separates left and right sides We need to solve for x . Return one of three possible answers: Case Return Exactly one solution "x=#value" No solution "No solution" Infinite solutions "Infinite solutions" If there is exactly one solution, the problem guarantees...
leetcode string math simulation parsing May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 609: Find Duplicate File in System

Problem Restatement We are given a list of directory information strings. Each string has this format: "directory_path file1.txt(content1) file2.txt(content2) ..." For example: "root/a 1.txt(abcd) 2.txt(efgh)" means directory root/a contains: File Content 1.txt abcd 2.txt efgh We need to return groups of duplicate files. Two files are duplicates if they have exactly the same content, even if they have different names or live in different directories. Each returned group must contain...
leetcode array hash-table string parsing May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 592: Fraction Addition and Subtraction

Problem Restatement We are given a string expression representing additions and subtractions of fractions. Each fraction has the form: numerator/denominator A fraction may have a leading + or - . We need to evaluate the whole expression and return the final result as an irreducible fraction string. If the result is an integer, we still return it with denominator 1 . For example: 2 must be returned as: "2/1" The...
leetcode math string parsing gcd May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 591: Tag Validator

Problem Restatement We are given a string called code . We must determine whether it is a valid code snippet according to several XML-like rules. The string may contain: Start tags: <TAG> End tags: </TAG> CDATA sections: <![CDATA[ ... ]]> Rules: Tag names must contain only uppercase English letters. Tag name length must be between 1 and 9 . Tags must be properly nested. The entire code must be wrapped...
leetcode stack string parsing May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 726: Number of Atoms

Problem Restatement We are given a valid chemical formula string. The formula contains atom names, numbers, and parentheses. We need to count how many times each atom appears, then return the result as a string sorted by atom name. An atom name starts with one uppercase letter. It may be followed by lowercase letters. Examples: "H" "Mg" "Ca" A number after an atom means the atom appears that many times....
leetcode string stack hash-map sorting parsing May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 536: Construct Binary Tree from String

Problem Restatement We are given a string representing a binary tree. The string format is: node(left_subtree)(right_subtree) Each node starts with an integer value. If a node has children, the left subtree appears first inside parentheses. The right subtree appears second inside another pair of parentheses. We need to reconstruct the binary tree and return its root. The string may contain negative integers. Input and Output Item Meaning Input A string...
leetcode tree binary-tree recursion string parsing May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 00xx ›

LeetCode 65: Valid Number

Problem Restatement We are given a string s . We need to return true if s represents a valid number. Otherwise, return false . A valid number can have: A decimal number or an integer Optionally, an exponent part using e or E , followed by an integer The official examples include valid strings such as: "2" "0089" "-0.1" "+3.14" "4." "-.9" "2e10" "-90E3" "3e+7" "+6e-1" "53.5e93" "-123.456e789" And invalid...
leetcode string finite-state-machine parsing May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 439: Ternary Expression Parser

Problem Restatement We are given a valid string expression representing nested ternary expressions. A ternary expression has the form: condition ? true_value : false_value The condition is always either: T F where T means true and F means false. The expression contains only: Character Meaning T true F false 0 to 9 single digit value ? starts a ternary choice : separates true branch and false branch Each number has...
leetcode string stack parsing May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 227: Basic Calculator II

Problem Restatement We are given a string s representing a mathematical expression. The expression contains: Non-negative integers Operators: + , - , * , / Spaces We must evaluate the expression and return the integer result. Division truncates toward zero. LeetCode examples: Input: "3+2*2" Output: 7 Input: " 3/2 " Output: 1 Input: " 3+5 / 2 " Output: 5 The expression is always valid. Input and Output Item Meaning...
leetcode stack string parsing math May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 00xx ›

LeetCode 8: String to Integer (atoi)

Problem Restatement We are given a string s . We need to implement myAtoi(s) , which converts the beginning of the string into a signed 32-bit integer. The parsing follows these rules: Ignore leading spaces. Read an optional sign, either + or - . Read digits until a non-digit character appears. If no digits are read, return 0 . Clamp the result into the signed 32-bit integer range. The signed...
leetcode string parsing simulation May 10, 2026