Create basic-calculator-iii.cpp

C++/basic-calculator-iii.cpp

@@ -0,0 +1,59 @@
+// Time:  O(n)
+// Space: O(n)
+
+// Support +, -, *, /.
+class Solution {
+public:
+    int calculate(string s) {
+        stack<int64_t> operands;
+        stack<char> operators;
+        string operand;
+        for (int i = s.length() - 1; i >= 0; --i) {
+            if (isdigit(s[i])) {
+                operand.push_back(s[i]);
+                if (i == 0 || !isdigit(s[i - 1])) {
+                    reverse(operand.begin(), operand.end());
+                    operands.emplace(stol(operand));
+                    operand.clear();
+                }
+            } else if (s[i] == ')' || s[i] == '*' ||
+                       s[i] == '/') {
+                operators.emplace(s[i]);
+            } else if (s[i] == '+' || s[i] == '-') {
+                while (!operators.empty() && (operators.top() == '*' ||
+                       operators.top() == '/')) {
+                     compute(operands, operators);
+                }
+                operators.emplace(s[i]);
+            } else if (s[i] == '(') {
+                // operators at least one element, i.e. ')'.
+                while (operators.top() != ')') {
+                    compute(operands, operators);
+                }
+                operators.pop();
+            }
+        }
+        while (!operators.empty()) {
+            compute(operands, operators);
+        }
+        return operands.top();
+    }
+
+    void compute(stack<int64_t>& operands, stack<char>& operators) {
+        const int64_t left = operands.top();
+        operands.pop();
+        const int64_t right = operands.top();
+        operands.pop();
+        const char op = operators.top();
+        operators.pop();
+        if (op == '+') {
+            operands.emplace(left + right);
+        } else if (op == '-') {
+            operands.emplace(left - right);
+        } else if (op == '*') {
+            operands.emplace(left * right);
+        } else if (op == '/') {
+            operands.emplace(left / right);
+        }
+    }
+};