224. Basic Calculator

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224. Basic Calculator

Description

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Example 1:

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Input: s = "1 + 1"
Output: 2

Example 2:

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Input: s = " 2-1 + 2 "
Output: 3

Example 3:

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Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23

Constraints:

  • 1 <= s.length <= 3 * 10^5
  • s consists of digits, '+', '-', '(', ')', and ' '.
  • s represents a valid expression.
  • '+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).
  • '-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).
  • There will be no two consecutive operators in the input.
  • Every number and running calculation will fit in a signed 32-bit integer.

Hints/Notes

Solution

Language: C++

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class Solution {
public:
    int calculate(string s) {
        s = '(' + s + ')';
        stack<string> stk;
        for (int i = 0; i < s.size(); ) {
            if (s[i] != ' ') {
                if (s[i] == '(' || s[i] == '+' || s[i] == '-') {
                    string tmp{s[i]};
                    stk.push(tmp);
                    i++;
                } else if (isdigit(s[i])) {
                    int cur = 0;
                    while (isdigit(s[i])) {
                        cur = cur * 10 + (s[i++] - '0');
                    }
                    stk.push(to_string(cur));
                } else {
                    int sum = 0, cur = 0;
                    while (stk.top() != "(") {
                        string c = stk.top();
                        stk.pop();
                        if (c == "+") {
                            sum += cur;
                            cur = 0;
                        } else if (c == "-"){
                            sum -= cur;
                            cur = 0;
                        } else {
                            cur = stoi(c);
                        }
                    }
                    i++;
                    sum += cur;
                    stk.pop();
                    stk.push(to_string(sum));
                }
            } else {
                i++;
            }
        }
        return stoi(stk.top());
    }
};