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86 lines
2.6 KiB
86 lines
2.6 KiB
#include <bits/stdc++.h>
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using namespace std;
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// 测试用例：
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// (23+124)*(98-2)
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// 中缀转后缀
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unordered_map<char, int> g{{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}}; // 操作符优化级
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string infixToPostfix(string s) {
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stack<char> stk; // 栈
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string t; // 结果串
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for (int i = 0; i < s.size(); i++) {
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// ①数字
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if (isdigit(s[i])) {
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int x = 0;
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while (i < s.size() && isdigit(s[i])) {
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x = x * 10 + s[i] - '0';
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i++;
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}
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i--;
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t += to_string(x) + " ";
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} else if (isalpha(s[i])) // ②字符,比如a,b,c
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t.push_back(s[i]), t.push_back(' ');
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else if (s[i] == '(') // ③左括号
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stk.push(s[i]); // 左括号入栈
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else if (s[i] == ')') { // ④右括号
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while (stk.top() != '(') { // 让栈中元素(也就是+-*/和左括号)一直出栈，直到匹配的左括号出栈
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t.push_back(stk.top()), t.push_back(' ');
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stk.pop();
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}
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stk.pop(); // 左括号也需要出栈
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} else {
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// ⑤操作符 +-*/
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while (stk.size() && g[s[i]] <= g[stk.top()]) {
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t.push_back(stk.top()), t.push_back(' ');
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stk.pop();
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}
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stk.push(s[i]); // 将自己入栈
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}
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}
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// 当栈不为空时，全部输出
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while (stk.size()) {
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t.push_back(stk.top()), t.push_back(' ');
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stk.pop();
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}
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return t;
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}
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// 判断字符是否是操作符
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bool isOp(char c) {
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return c == '+' || c == '-' || c == '*' || c == '/';
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}
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// 后缀表达式求值
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int eval(string s) {
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stack<int> stk; // 后缀表达式求值要使用到一个栈
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// 构造一个字符串流，方便处理带空格的后缀表达式
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stringstream s1(s);
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string c;
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while (s1 >> c) {
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if (isOp(c[0])) { // 如果是操作数，将其转换为整数并入栈
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int a = stk.top();
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stk.pop();
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int b = stk.top();
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stk.pop();
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if (c == "+") stk.push(b + a);
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if (c == "-") stk.push(b - a);
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if (c == "*") stk.push(b * a);
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if (c == "/") stk.push(b / a);
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} else
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stk.push(stoi(c));
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}
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return stk.top();
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}
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int main() {
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string s;
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cin >> s;
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// 中缀转后缀
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s = infixToPostfix(s);
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// 后缀表达式求值
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cout << eval(s) << endl;
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return 0;
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} |