✅ 中缀转后缀+表达式计算

simple-calculator/main.cc

@@ -0,0 +1,76 @@
+#include <cmath>
+#include <iostream>
+#include <stack>
+#include <unordered_map>
+using namespace std;
+
+unordered_map<char, int> priority{{'(', 0}, {')', 0}, {'+', 1}, {'-', 1},
+                                  {'*', 2}, {'/', 2}, {'^', 3}};
+
+string in_order2post_order(string src) {
+  stack<char> in;
+  string builder;
+  for (int i = 0; i < src.size(); i++) {
+    if (src[i] == ' ')
+      continue;
+    if (isdigit(src[i]))
+      builder += src[i];
+    else {
+      builder += ' ';
+      while (!in.empty() && priority[in.top()] >= priority[src[i]]) {
+        builder += in.top();
+        in.pop();
+      }
+      in.push(src[i]);
+    }
+  }
+  if (builder[builder.size() - 1] != ' ')
+    builder += ' ';
+  while (!in.empty()) {
+    builder += in.top();
+    in.pop();
+  }
+  return builder;
+}
+
+int eval(string src) {
+  stack<int> in;
+  int num_builder = 0;
+  for (int i = 0; i < src.size(); i++) {
+    if (isdigit(src[i])) {
+      num_builder = num_builder * 10 + src[i] - '0';
+    } else if (src[i] == ' ') {
+      in.push(num_builder);
+      num_builder = 0;
+    } else {
+      int y = in.top();
+      in.pop();
+      int x = in.top();
+      in.pop();
+      switch (src[i]) {
+      case '+':
+        in.push(x + y);
+        break;
+      case '-':
+        in.push(x - y);
+        break;
+      case '*':
+        in.push(x * y);
+        break;
+      case '/':
+        in.push(x / y);
+        break;
+      case '^':
+        in.push(pow(x, y));
+        break;
+      }
+    }
+  }
+  return in.top();
+}
+
+int main() {
+  string str;
+  getline(cin, str);
+  cout << eval(in_order2post_order(str)) << endl;
+}