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224. Basic Calculator

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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:

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.

Solutions

Solution 1: Stack

We use a stack stk to save the current calculation result and operator, a variable sign to save the current sign, and a variable ans to save the final calculation result.

Next, we traverse each character of the string s:

If the current character is a number, we use a loop to read the following consecutive numbers, and then add or subtract it to ans according to the current sign.
If the current character is '+', we change the variable sign to positive.
If the current character is '-', we change the variable sign to negative.
If the current character is '(', we push the current ans and sign into the stack, and reset them to empty and 1, and start to calculate the new ans and sign.
If the current character is ')', we pop the top two elements of the stack, one is the operator, and the other is the number calculated before the bracket. We multiply the current number by the operator, and add the previous number to get the new ans.

After traversing the string s, we return ans.

The time complexity is O(n), and the space complexity is O(n), where n is the length of the string s.

Python3

class Solution:
    def calculate(self, s: str) -> int:
        stk = []
        ans, sign = 0, 1
        i, n = 0, len(s)
        while i < n:
            if s[i].isdigit():
                x = 0
                j = i
                while j < n and s[j].isdigit():
                    x = x * 10 + int(s[j])
                    j += 1
                ans += sign * x
                i = j - 1
            elif s[i] == "+":
                sign = 1
            elif s[i] == "-":
                sign = -1
            elif s[i] == "(":
                stk.append(ans)
                stk.append(sign)
                ans, sign = 0, 1
            elif s[i] == ")":
                ans = stk.pop() * ans + stk.pop()
            i += 1
        return ans

Java

class Solution {
    public int calculate(String s) {
        Deque<Integer> stk = new ArrayDeque<>();
        int sign = 1;
        int ans = 0;
        int n = s.length();
        for (int i = 0; i < n; ++i) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                int j = i;
                int x = 0;
                while (j < n && Character.isDigit(s.charAt(j))) {
                    x = x * 10 + s.charAt(j) - '0';
                    j++;
                }
                ans += sign * x;
                i = j - 1;
            } else if (c == '+') {
                sign = 1;
            } else if (c == '-') {
                sign = -1;
            } else if (c == '(') {
                stk.push(ans);
                stk.push(sign);
                ans = 0;
                sign = 1;
            } else if (c == ')') {
                ans = stk.pop() * ans + stk.pop();
            }
        }
        return ans;
    }
}

C++

class Solution {
public:
    int calculate(string s) {
        stack<int> stk;
        int ans = 0, sign = 1;
        int n = s.size();
        for (int i = 0; i < n; ++i) {
            if (isdigit(s[i])) {
                int x = 0;
                int j = i;
                while (j < n && isdigit(s[j])) {
                    x = x * 10 + (s[j] - '0');
                    ++j;
                }
                ans += sign * x;
                i = j - 1;
            } else if (s[i] == '+') {
                sign = 1;
            } else if (s[i] == '-') {
                sign = -1;
            } else if (s[i] == '(') {
                stk.push(ans);
                stk.push(sign);
                ans = 0;
                sign = 1;
            } else if (s[i] == ')') {
                ans *= stk.top();
                stk.pop();
                ans += stk.top();
                stk.pop();
            }
        }
        return ans;
    }
};

Go

func calculate(s string) (ans int) {
	stk := []int{}
	sign := 1
	n := len(s)
	for i := 0; i < n; i++ {
		switch s[i] {
		case ' ':
		case '+':
			sign = 1
		case '-':
			sign = -1
		case '(':
			stk = append(stk, ans)
			stk = append(stk, sign)
			ans, sign = 0, 1
		case ')':
			ans *= stk[len(stk)-1]
			stk = stk[:len(stk)-1]
			ans += stk[len(stk)-1]
			stk = stk[:len(stk)-1]
		default:
			x := 0
			j := i
			for ; j < n && '0' <= s[j] && s[j] <= '9'; j++ {
				x = x*10 + int(s[j]-'0')
			}
			ans += sign * x
			i = j - 1
		}
	}
	return
}

TypeScript

function calculate(s: string): number {
    const stk: number[] = [];
    let sign = 1;
    let ans = 0;
    const n = s.length;
    for (let i = 0; i < n; ++i) {
        if (s[i] === ' ') {
            continue;
        }
        if (s[i] === '+') {
            sign = 1;
        } else if (s[i] === '-') {
            sign = -1;
        } else if (s[i] === '(') {
            stk.push(ans);
            stk.push(sign);
            ans = 0;
            sign = 1;
        } else if (s[i] === ')') {
            ans *= stk.pop() as number;
            ans += stk.pop() as number;
        } else {
            let x = 0;
            let j = i;
            for (; j < n && !isNaN(Number(s[j])) && s[j] !== ' '; ++j) {
                x = x * 10 + (s[j].charCodeAt(0) - '0'.charCodeAt(0));
            }
            ans += sign * x;
            i = j - 1;
        }
    }
    return ans;
}

C#

public class Solution {
    public int Calculate(string s) {
        var stk = new Stack<int>();
        int sign = 1;
        int n = s.Length;
        int ans = 0;
        for (int i = 0; i < n; ++i) {
            if (s[i] == ' ') {
                continue;
            }
            if (s[i] == '+') {
                sign = 1;
            } else if (s[i] == '-') {
                sign = -1;
            } else if (s[i] == '(') {
                stk.Push(ans);
                stk.Push(sign);
                ans = 0;
                sign = 1;
            } else if (s[i] == ')') {
                ans *= stk.Pop();
                ans += stk.Pop();
            } else {
                int num = 0;
                while (i < n && char.IsDigit(s[i])) {
                    num = num * 10 + s[i] - '0';
                    ++i;
                }
                --i;
                ans += sign * num;
            }
        }
        return ans;
    }
}