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Problem solution · C++

Number of Atoms

Number of Atoms: a C++ solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
53 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Number of Atoms, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 53 lines of C++ from the credited upstream file 726.cpp.
  • The implementation visibly relies on ordered lookup.
  • 6 loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeNumber of Atoms · C++C++
Use this to learn the idea, then write your own version.
class Solution { public:  string countOfAtoms(string formula) {    string ans;    int i = 0;     for (const auto& [elem, freq] : parse(formula, i)) {      ans += elem;      if (freq > 1)        ans += to_string(freq);    }     return ans;  }  private:  map<string, int> parse(const string& s, int& i) {    map<string, int> count;     while (i < s.length())      if (s[i] == '(') {        for (const auto& [elem, freq] : parse(s, ++i))          count[elem] += freq;      } else if (s[i] == ')') {        const int num = getNum(s, ++i);        for (auto&& [_, freq] : count)          freq *= num;        return count;  // Return back to the previous scope.      } else {         // `s[i]` must be uppercased.        const string& elem = getElem(s, i);        const int num = getNum(s, i);        count[elem] += num;      }     return count;  }   string getElem(const string& s, int& i) {    const int elemStart = i++;  // `s[elemStart]` is uppercased.    while (i < s.length() && islower(s[i]))      ++i;    return s.substr(elemStart, i - elemStart);  }   int getNum(const string& s, int& i) {    const int numStart = i;    while (i < s.length() && isdigit(s[i]))      ++i;    const string& numString = s.substr(numStart, i - numStart);    return numString.empty() ? 1 : stoi(numString);  }};

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