224. Basic Calculator

📋 Đề Bài

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:

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

Example 2:

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

Example 3:

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

Constraints:

1 <= s.length <= 3 * 10⁵
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.

🧠 Thuật Toán & Kỹ Thuật

Stack (Ngăn xếp)String (Chuỗi)

⏱️ Thời gian O(n)

💾 Không gian O(1)

💻 Lời Giải

C++ 0224-basic-calculator.cpp

class Solution {
public:
    int calculate(string s) {
        stack<int> st;
        st.push(1);
        int res = 0;
        int num = 0;
        int sgn = 1;
        for (char c : s) {
            if (isdigit(c)) {
                num = num * 10 + (c - '0');
            } 
            else if (c == '-' or c == '+') {
                res += num * sgn * st.top();
                sgn = c == '-' ? -1 : 1;
                num = 0;
            }
            else if (c == '(') {
                st.push(sgn * st.top());
                sgn = 1;
            }
            else if (c == ')') {
                res += num * sgn * st.top();
                num = 0;
                st.pop();
            }
        }
        res += num * sgn * st.top();
        return res;
    }
};