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Problem solution · C++

Basic Calculator IV

Basic Calculator IV: a C++ solution using stack-based processing. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Stack-based processing
Source
walkccc LeetCode Solutions
Length
197 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

Stack-based processing

For Basic Calculator IV, the implementation keeps unresolved items in last-in, first-out order, often to match boundaries, parse structure, or maintain monotonic candidates.

  1. Define what every stack entry represents.
  2. Pop entries once the current item resolves or invalidates them.
  3. Push the current item with only the information later steps need.

Code notes

  • 197 lines of C++ from the credited upstream file 770.cpp.
  • The implementation visibly relies on sequence storage, hash lookup.
  • 19 loop blocks detected.

Complexity

If each item is pushed and popped at most once, the stack work is linear.

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 codeBasic Calculator IV · C++C++
Use this to learn the idea, then write your own version.
class Poly {  friend Poly operator+(const Poly& lhs, const Poly& rhs) {    Poly res(lhs);    for (const auto& [term, coef] : rhs.terms)      res.terms[term] += coef;    return res;  }   friend Poly operator-(const Poly& lhs, const Poly& rhs) {    Poly res(lhs);    for (const auto& [term, coef] : rhs.terms)      res.terms[term] -= coef;    return res;  }   friend Poly operator*(const Poly& lhs, const Poly& rhs) {    Poly res;    for (const auto& [a, aCoef] : lhs.terms)      for (const auto& [b, bCoef] : rhs.terms)        res.terms[merge(a, b)] += aCoef * bCoef;    return res;  }   // Friend ostream& operator<<(ostream& os, const Poly& poly) {  //   os << "{";  //   for (const auto& [term, coef] : poly.terms)  //     os << term << ": " << coef << ", ";  //   os << "}";  //   return os;  // }  public:  vector<string> toList() {    vector<string> res;    vector<string> keys;    for (const auto& [term, _] : terms)      keys.push_back(term);    ranges::sort(keys, [&](const string& a, const string& b) {      // the minimum degree is the last      if (a == "1")        return false;      if (b == "1")        return true;      const vector<string> as = split(a, '*');      const vector<string> bs = split(b, '*');      // the maximum degree is the first      // Break ties by their lexicographic orders.      return as.size() == bs.size() ? a < b : as.size() > bs.size();    });    auto concat = [&](const string& term) -> string {      if (term == "1")        return to_string(terms[term]);      return to_string(terms[term]) + '*' + term;    };    for (const string& key : keys)      if (terms[key])        res.push_back(concat(key));    return res;  }   Poly() = default;  Poly(const string& term, int coef) {    terms[term] = coef;  }  private:  unordered_map<string, int> terms;   // e.g. merge("a*b", "a*c") -> "a*a*b*c"  static string merge(const string& a, const string& b) {    if (a == "1")      return b;    if (b == "1")      return a;    string res;    vector<string> A = split(a, '*');    vector<string> B = split(b, '*');    int i = 0;  // A's index    int j = 0;  // B's index    while (i < A.size() && j < B.size())      if (A[i] < B[j])        res += '*' + A[i++];      else        res += '*' + B[j++];    while (i < A.size())      res += '*' + A[i++];    while (j < B.size())      res += '*' + B[j++];    return res.substr(1);  }   static vector<string> split(const string& token, char c) {    vector<string> vars;    istringstream iss(token);    for (string var; getline(iss, var, c);)      vars.push_back(var);    return vars;  }}; class Solution { public:  vector<string> basicCalculatorIV(string expression, vector<string>& evalvars,                                   vector<int>& evalints) {    vector<string> tokens = getTokens(expression);    unordered_map<string, int> evalMap;     for (int i = 0; i < evalvars.size(); ++i)      evalMap[evalvars[i]] = evalints[i];     for (string& token : tokens)      if (const auto it = evalMap.find(token); it != evalMap.cend())        token = to_string(it->second);     const vector<string>& postfix = infixToPostfix(tokens);    return evaluate(postfix).toList();  }  private:  vector<string> getTokens(const string& s) {    vector<string> tokens;    int i = 0;    for (int j = 0; j < s.length(); ++j)      if (s[j] == ' ') {        if (i < j)          tokens.push_back(s.substr(i, j - i));        i = j + 1;      } else if (string("()+-*").find(s[j]) != string::npos) {        if (i < j)          tokens.push_back(s.substr(i, j - i));        tokens.push_back(s.substr(j, 1));        i = j + 1;      }    if (i < s.length())      tokens.push_back(s.substr(i));    return tokens;  }   bool isOperator(const string& token) {    return token == "+" || token == "-" || token == "*";  }   vector<string> infixToPostfix(const vector<string>& tokens) {    vector<string> postfix;    stack<string> ops;     auto precedes = [](const string& prevOp, const string& currOp) -> bool {      if (prevOp == "(")        return false;      return prevOp == "*" || currOp == "+" || currOp == "-";    };     for (const string& token : tokens)      if (token == "(") {        ops.push(token);      } else if (token == ")") {        while (ops.top() != "(")          postfix.push_back(ops.top()), ops.pop();        ops.pop();      } else if (isOperator(token)) {        while (!ops.empty() && precedes(ops.top(), token))          postfix.push_back(ops.top()), ops.pop();        ops.push(token);      } else {  // isOperand(token)        postfix.push_back(token);      }     while (!ops.empty())      postfix.push_back(ops.top()), ops.pop();     return postfix;  }   Poly evaluate(const vector<string>& postfix) {    vector<Poly> polys;    for (const string& token : postfix)      if (isOperator(token)) {        const Poly b = polys.back();        polys.pop_back();        const Poly a = polys.back();        polys.pop_back();        if (token == "+")          polys.push_back(a + b);        else if (token == "-")          polys.push_back(a - b);        else  // token == "*"          polys.push_back(a * b);      } else if (token[0] == '-' ||                 ranges::all_of(token, [](char c) { return isdigit(c); })) {        polys.push_back(Poly("1", stoi(token)));      } else {        polys.push_back(Poly(token, 1));      }    return polys[0];  }};

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