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Problem solution · Java

Basic Calculator IV

Basic Calculator IV: a Java 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
183 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

  • 183 lines of Java from the credited upstream file 770.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup, work queue.
  • 17 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 · JavaJava
Use this to learn the idea, then write your own version.
class Poly {  public Poly add(Poly o) {    for (final String term : o.terms.keySet())      terms.merge(term, o.terms.get(term), Integer::sum);    return this;  }   public Poly minus(Poly o) {    for (final String term : o.terms.keySet())      terms.merge(term, -o.terms.get(term), Integer::sum);    return this;  }   public Poly mult(Poly o) {    Poly res = new Poly();    for (final String a : terms.keySet())      for (final String b : o.terms.keySet())        res.terms.merge(merge(a, b), terms.get(a) * o.terms.get(b), Integer::sum);    return res;  }   // @Override  // Public String toString() {  //   StringBuilder sb = new StringBuilder();  //   sb.append("{");  //   for (final String term : terms.keySet())  //     sb.append(term).append(": ").append(terms.get(term)).append(", ");  //   sb.append("}");  //   return sb.toString();  // }   public List<String> toList() {    List<String> res = new ArrayList<>();    List<String> keys = new ArrayList<>(terms.keySet());    Collections.sort(keys, new Comparator<String>() {      @Override      public int compare(final String a, final String b) {        // the minimum degree is the last        if (a.equals("1"))          return 1;        if (b.equals("1"))          return -1;        String[] as = a.split("\\*");        String[] bs = b.split("\\*");        // the maximum degree is the first        // Break ties by their lexicographic orders.        return as.length == bs.length ? a.compareTo(b) : bs.length - as.length;      }    });    for (final String key : keys)      if (terms.get(key) != 0)        res.add(concat(key));    return res;  }   public Poly() {}  public Poly(final String term, int coef) {    terms.put(term, coef);  }   private Map<String, Integer> terms = new HashMap<>();   // e.g. merge("a*b", "a*c") -> "a*a*b*c"  private static String merge(final String a, final String b) {    if (a.equals("1"))      return b;    if (b.equals("1"))      return a;    StringBuilder sb = new StringBuilder();    String[] A = a.split("\\*");    String[] B = b.split("\\*");    int i = 0; // A's index    int j = 0; // B's index    while (i < A.length && j < B.length)      if (A[i].compareTo(B[j]) < 0)        sb.append("*").append(A[i++]);      else        sb.append("*").append(B[j++]);    while (i < A.length)      sb.append("*").append(A[i++]);    while (j < B.length)      sb.append("*").append(B[j++]);    return sb.substring(1).toString();  }   private String concat(final String term) {    if (term.equals("1"))      return String.valueOf(terms.get(term));    return new StringBuilder().append(terms.get(term)).append('*').append(term).toString();  }} class Solution {  public List<String> basicCalculatorIV(String expression, String[] evalvars, int[] evalints) {    List<String> tokens = getTokens(expression);    Map<String, Integer> evalMap = new HashMap<>();     for (int i = 0; i < evalvars.length; ++i)      evalMap.put(evalvars[i], evalints[i]);     for (int i = 0; i < tokens.size(); ++i)      if (evalMap.containsKey(tokens.get(i)))        tokens.set(i, String.valueOf(evalMap.get(tokens.get(i))));     List<String> postfix = infixToPostfix(tokens);    return evaluate(postfix).toList();  }   private List<String> getTokens(final String s) {    List<String> tokens = new ArrayList<>();    int i = 0;    for (int j = 0; j < s.length(); ++j)      if (s.charAt(j) == ' ') {        if (i < j)          tokens.add(s.substring(i, j));        i = j + 1;      } else if ("()+-*".contains(s.substring(j, j + 1))) {        if (i < j)          tokens.add(s.substring(i, j));        tokens.add(s.substring(j, j + 1));        i = j + 1;      }    if (i < s.length())      tokens.add(s.substring(i));    return tokens;  }   private boolean isOperator(final String token) {    return token.equals("+") || token.equals("-") || token.equals("*");  }   private boolean precedes(final String prevOp, final String currOp) {    if (prevOp.equals("("))      return false;    return prevOp.equals("*") || currOp.equals("+") || currOp.equals("-");  }   private List<String> infixToPostfix(List<String> tokens) {    List<String> postfix = new ArrayList<>();    Deque<String> ops = new ArrayDeque<>();     for (final String token : tokens)      if (token.equals("(")) {        ops.push(token);      } else if (token.equals(")")) {        while (!ops.peek().equals("("))          postfix.add(ops.pop());        ops.pop();      } else if (isOperator(token)) {        while (!ops.isEmpty() && precedes(ops.peek(), token))          postfix.add(ops.pop());        ops.push(token);      } else { // isOperand(token)        postfix.add(token);      }     while (!ops.isEmpty())      postfix.add(ops.pop());     return postfix;  }   private Poly evaluate(List<String> postfix) {    LinkedList<Poly> polys = new LinkedList<>();    for (final String token : postfix)      if (isOperator(token)) {        final Poly b = polys.removeLast();        final Poly a = polys.removeLast();        if (token.equals("+"))          polys.add(a.add(b));        else if (token.equals("-"))          polys.add(a.minus(b));        else // token == "*"          polys.add(a.mult(b));      } else if (token.charAt(0) == '-' || token.chars().allMatch(c -> Character.isDigit(c))) {        polys.add(new Poly("1", Integer.parseInt(token)));      } else {        polys.add(new Poly(token, 1));      }    return polys.getFirst();  }}

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