[LeetCode] 227. Basic Calculator II 基本计算器 II-优快云博客

本文介绍了一个简单的表达式计算器实现，该计算器能够解析并计算包含非负整数、加减乘除运算符及空格的字符串表达式。文章提供了Java、Python和C++三种语言的示例代码，并强调了不得使用内置的eval函数。

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

Java:

public int calculate(String s) {
    int md=-1; // 0 is m, 1 is d
    int sign=1; // 1 is +, -1 is -
    int prev=0;
    int result=0;
 
    for(int i=0; i<s.length(); i++){
        char c = s.charAt(i);
        if(Character.isDigit(c)){
            int num = c-'0';
            while(++i<s.length() && Character.isDigit(s.charAt(i))){
                num = num*10+s.charAt(i)-'0';
            }
            i--; // back to last digit of number
 
            if(md==0){
                prev = prev * num;
                md=-1;
            }else if(md==1){
                prev = prev / num;
                md=-1;
            }else{
                prev = num;
            }
        }else if(c=='/'){
            md=1;
        }else if(c=='*'){
            md=0;
        }else if(c=='+'){
            result = result + sign*prev;
            sign=1;
        }else if(c=='-'){
            result = result + sign*prev;
            sign=-1;
        }
    }
 
    result = result + sign*prev;
    return result;
}

Python:

class Solution:
    # @param {string} s
    # @return {integer}
    def calculate(self, s):
        operands, operators = [], []
        operand = ""
        for i in reversed(xrange(len(s))):
            if s[i].isdigit():
                operand += s[i]
                if i == 0  or not s[i-1].isdigit():
                    operands.append(int(operand[::-1]))
                    operand = ""
            elif s[i] == ')' or s[i] == '*' or s[i] == '/':
                operators.append(s[i])
            elif s[i] == '+' or s[i] == '-':
                while operators and \
                      (operators[-1] == '*' or operators[-1] == '/'):
                    self.compute(operands, operators)
                operators.append(s[i])
            elif s[i] == '(':
                while operators[-1] != ')':
                    self.compute(operands, operators)
                operators.pop()
                
        while operators:
            self.compute(operands, operators)
            
        return operands[-1]

    def compute(self, operands, operators):
        left, right = operands.pop(), operands.pop()
        op = operators.pop()
        if op == '+':
            operands.append(left + right)
        elif op == '-':
            operands.append(left - right)
        elif op == '*':
            operands.append(left * right)
        elif op == '/':
            operands.append(left / right)

C++:

class Solution {
public:
    int calculate(string s) {
        int res = 0, d = 0;
        char sign = '+';
        stack<int> nums;
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] >= '0') {
                d = d * 10 + s[i] - '0';
            }
            if ((s[i] < '0' && s[i] != ' ') || i == s.size() - 1) {
                if (sign == '+') nums.push(d);
                if (sign == '-') nums.push(-d);
                if (sign == '*' || sign == '/') {
                    int tmp = sign == '*' ? nums.top() * d : nums.top() / d;
                    nums.pop();
                    nums.push(tmp);
                }
                sign = s[i];
                d = 0;
            } 
        }
        while (!nums.empty()) {
            res += nums.top();
            nums.pop();
        }
        return res;
    }
};

C++:　　

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);
        }
    }
};

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[LeetCode] 224. Basic Calculator 基本计算器