227 Basic Calculator II

1 题目

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.Example 1:Input: "3+2*2"
Output: 7
Example 2:Input: " 3/2 "
Output: 1
Example 3:Input: " 3+5 / 2 "
Output: 5
Note:You may assume that the given expression is always valid.
Do not use the eval built-in library function.

2 尝试解

class Solution {
public:int calculate(string s) {s = '+' + s;stack<int> num;map<char,int> order{{'+',1},{'-',1},{'*',2},{'/',2}};for(int i = 0; i < s.size(); i++){if(s[i] ==' ') continue;if(order.count(s[i])!=0){char op = s[i];i++;string temp = "";while(s[i] == ' ' ||(s[i] >= '0' && s[i] <= '9')){temp+=s[i];i++;}i--;int num2 = atoi(temp.c_str());switch (op){case '+': num.push(num2); break;case '-': num.push(-num2); break;case '*': num.top() *= num2; break;case '/': num.top() /= num2; break;}}}int result = 0;while(!num.empty()){result += num.top();num.pop();}return result;}
};

3 标准解

int calculate(string s) {istringstream in('+' + s + '+');long long total = 0, term = 0, n;char op;while (in >> op) {if (op == '+' or op == '-') {total += term;in >> term;term *= 44 - op;} else {in >> n;if (op == '*')term *= n;elseterm /= n;}}return total;
}

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