4kyu: add infix to postfix converter

Signed-off-by: Matej Focko <mfocko@redhat.com>

4kyu/infix_to_postfix_converter/solution.cpp

@@ -0,0 +1,108 @@
+#include <cassert>
+#include <string>
+#include <vector>
+
+namespace {
+
+template <typename T>
+void pop_and_push(std::vector<T> &src, std::vector<T> &dst) {
+  assert(src.size());
+  dst.push_back(src.back());
+  src.pop_back();
+}
+
+int priority(char op) {
+  switch (op) {
+  case '(':
+  case ')':
+    return 0;
+  case '+':
+  case '-':
+    return 1;
+  case '*':
+  case '/':
+    return 2;
+  case '^':
+    return 3;
+  default:
+    assert(false);
+  }
+}
+
+enum assoc_t { LEFT, RIGHT, NONE };
+
+assoc_t associativity(char op) {
+  assert(!std::isdigit(op));
+
+  switch (op) {
+  case '(':
+  case ')':
+    return assoc_t::NONE;
+  case '^':
+    return assoc_t::RIGHT;
+  default:
+    return assoc_t::LEFT;
+  }
+}
+
+bool pop_operator(std::vector<char> &op_stack, char o1) {
+  if (op_stack.empty()) {
+    return false;
+  }
+
+  char o2 = op_stack.back();
+  if (o2 == '(') {
+    return false;
+  }
+
+  int o1_priority = priority(o1);
+  int o2_priority = priority(o2);
+
+  return o2_priority > o1_priority ||
+         (o2_priority == o1_priority && associativity(o1) == assoc_t::LEFT);
+}
+
+} // namespace
+
+std::string to_postfix(std::string infix) {
+  std::vector<char> output_queue;
+  std::vector<char> operator_stack;
+
+  for (char token : infix) {
+    if (std::isdigit(token)) {
+      output_queue.push_back(token);
+    } else if (token == '(') {
+      operator_stack.push_back(token);
+    } else if (token == ')') {
+      while (operator_stack.back() != '(') {
+        assert(operator_stack.size());
+        pop_and_push(operator_stack, output_queue);
+      }
+
+      assert(operator_stack.back() == '(');
+      operator_stack.pop_back();
+    } else {
+      while (pop_operator(operator_stack, token)) {
+        pop_and_push(operator_stack, output_queue);
+      }
+      operator_stack.push_back(token);
+    }
+  }
+
+  while (operator_stack.size()) {
+    assert(operator_stack.back() != '(');
+    pop_and_push(operator_stack, output_queue);
+  }
+
+  return std::string(output_queue.begin(), output_queue.end());
+}
+
+int main() {
+  assert(to_postfix("2+7*5") == "275*+");
+  assert(to_postfix("3*3/(7+1)") == "33*71+/");
+  assert(to_postfix("5+(6-2)*9+3^(7-1)") == "562-9*+371-^+");
+  assert(to_postfix("(5-4-1)+9/5/2-7/1/7") == "54-1-95/2/+71/7/-");
+  assert(to_postfix("1^2^3") == "123^^");
+
+  return 0;
+}

README.md

@@ -38,6 +38,7 @@
 
 - [Permutations](https://www.codewars.com/kata/5254ca2719453dcc0b00027d) - [solution](4kyu/permutations)
 - [Matrix Determinant](https://www.codewars.com/kata/52a382ee44408cea2500074c) - [solution](4kyu/matrix_determinant)
+- [Infix to Postfix Converter](https://www.codewars.com/kata/52e864d1ffb6ac25db00017f) - [solution](4kyu/infix_to_postfix_converter)
 
 ### JS