LeetCode 224.Basic Calculator

问题

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

“1 + 1” = 2
” 2-1 + 2 ” = 3
“(1+(4+5+2)-3)+(6+8)” = 23

解析

class Solution {public://比较运算符的优先级int compare(char top, char curr){    if (top == '+' || top == '-'){        if (curr == '*' || curr == '/' || curr == '(')            return 1;    //返回1表示当前元素进栈        else            return 0;   //返回0表示弹出栈顶元素    }    else if (top == '*' || top == '/'){        if (curr == '(')            return 1;        else            return 0;    }    else{        return 1;    }}//将中缀表达式转换为后缀表达式vector<string> changeToPostfix(string s) {    vector<string> result;    stack<char> operators;    string number;    for (int i = 0; i < s.size(); i++){        if (s[i] >= '0'&&s[i] <= '9'){            number += s[i];        }        else{            if (number != ""){                result.push_back(number);                number = "";            }            if (s[i] == ' '){                continue;            }            if (s[i] != ')'){                while (!operators.empty() && !compare(operators.top(), s[i])){                    result.push_back(string(1,operators.top()));                    operators.pop();                }                operators.push(s[i]);            }            else{                while (operators.top() != '('){                    result.push_back(string(1,operators.top()));                    operators.pop();                }                operators.pop();            }        }    }    if (number != ""){        result.push_back(number);    }    while (!operators.empty()){        result.push_back(string(1,operators.top()));        operators.pop();    }    return  result;}//后缀表达式求值int evalRPN(vector<string>& tokens) {    stack<int>nums;    for (int i = 0; i<tokens.size(); i++){        if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/"){            int right = nums.top();            nums.pop();            int left = nums.top();            nums.pop();            if (tokens[i] == "+"){                nums.push(left + right);            }            else if (tokens[i] == "-"){                nums.push(left - right);            }            else if (tokens[i] == "*"){                nums.push(left*right);            }            else{                nums.push(left / right);            }        }        else{            nums.push(stoi(tokens[i]));        }    }    return nums.top();}int calculate(string s) {    vector<string> postfix=changeToPostfix(s);      return evalRPN(postfix);}};