题解 | #四则运算#

先把表达式中所有的括号去掉，定义一个函数，对于每一对括号，截取字符串对应位置的子字符串，然后去掉括号并处理其中的内容，如果还有括号则继续使用此函数处理，直到其中没有括号为止，将所有去掉括号后的字符串拼接起来，然后按照四则运算的先后顺序计算出结果。

#include <cstddef>
#include <iostream>
using namespace std;

string calBracket(string s);
string calResult(const string & s);

int main() {
    string s;
    cin >> s;
    s = calBracket(s);
    cout << calResult(s);
}

string calculate(string s1, string s2, char operation) {
    if (operation == '+') {
        return to_string(stoi(s1) + stoi(s2));
    } else if (operation == '-') {
        return to_string(stoi(s1) - stoi(s2));
    } else if (operation == '*') {
        return to_string(stoi(s1) * stoi(s2));
    } else if (operation == '/') {
        return to_string(stoi(s1) / stoi(s2));
    }
    return NULL;
}

string calResult(const string & s) {
    string ans;
    string temp;
    string temp2;
    char operation;
    for (int i = 0; i < s.length(); i++) {
        if (s.at(i) == '+' || s.at(i) == '-') {
            if (i > 0 && (s.at(i - 1) == '+' || s.at(i - 1) == '-' || s.at(i - 1) == '*' || s.at(i - 1) == '/')) {
                temp += s.at(i);
                continue;
            }
            if (operation == '*' || operation == '/') {
                temp = calculate(temp2, temp, operation);
            }
            operation = s.at(i);
            ans += temp;
            ans += s.at(i);
            temp.clear();
            temp2.clear();
            continue;
        } else if (s.at(i) == '*' || s.at(i) == '/') {
            if (!temp2.empty()) {
                temp2 = calculate(temp2, temp, operation);
            } else {
                temp2 = temp;
            }
            temp.clear();
            operation = s.at(i);
            continue;
        }
        temp += s.at(i);
        if (i == s.length() - 1) {
            if (operation == '*' || operation == '/') {
                temp = calculate(temp2, temp, operation);
            }
            ans += temp;
        }
    }
    temp.clear();
    temp2.clear();
    for (int i = 0; i < ans.length(); i++) {
        if (ans.at(i) == '+' || ans.at(i) == '-') {
            if (i > 0 && (ans.at(i - 1) == '+' || ans.at(i - 1) == '-') || i == 0) {
                temp += ans.at(i);
                continue;
            }
            if (!temp2.empty()) {
                temp2 = calculate(temp2, temp, operation);
            } else {
                temp2 = temp;
            }
            temp.clear();
            operation = ans.at(i);
            continue;
        }
        temp += ans.at(i);
        if (i == ans.length() - 1) {
            if (operation == '+' || operation == '-') {
                if (!temp2.empty()) {
                    temp = calculate(temp2, temp, operation);
                }
            }
        }
    }
    ans = temp;
    return ans;
}

string calBracket(string s) {
    string ans;
    int start;
    int end;
    while (s.find('(') != string::npos || s.find('[') != string::npos || s.find('{') != string::npos) {
        for (int i = 0; i < s.length(); i++) {
            if (s.at(i) == '{') {
                start = i + 1;
                end = s.find('}', start);
                ans += '[' + calBracket(s.substr(start, end - start)) + ']';
                i = end;
                continue;
            } else if (s.at(i) == '[') {
                start = i + 1;
                end = s.find(']', start);
                ans += '(' + calBracket(s.substr(start, end - start)) + ')';
                i = end;
                continue;
            } else if (s.at(i) == '(') {
                start = i + 1;
                int count = 1;
                for (int j = start; j < s.length(); j++) {
                    if (s.at(j) == '(') {
                        count++;
                    } else if (s.at(j) == ')') {
                        count--;
                    }
                    if (count == 0) {
                        count = j;
                        break;
                    }
                }
                end = count;
                if (s.substr(start, end - start).find('(') != string::npos) {
                    ans += '(' + calBracket(s.substr(start, end - start)) + ')';
                } else {
                    ans += calResult(s.substr(start, end - start));
                }
                i = end;
                continue;
            }
            ans += s.at(i);
        }
        s = ans;
        ans.clear();
    }
    return s;
}
// 64 位输出请用 printf("%lld")