中缀到后缀的程序不能产生正确的输出

Question

#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <iomanip>
using namespace std;
class Stack {
public:
string stack[100][1];
int size;
Stack() { size = 0; }
~Stack() {};
void push(string data) {
stack[size][0] = data;
size += 1;
}
bool empty() {
if (size == 0)
{
return true;
}
else
{
return false;
}
}
string pop() {
size -= 1;
return stack[size][0];
}
string top() {
if (empty())
cout << "Stack is empty";
return stack[size - 1][0];
}
};
int pre(char op)
{
switch (op)
{
case '(': case ')': return 0;
case '+': case '-': return 1;
case '*': case '/': return 2;
}
return -1;
}
vector<string> infix_to_postfix(const vector<string>& expr) {
vector<string> postfix;
string op;
Stack st;
for (int i = 0; i < expr.size(); i++) {
if (expr[i][0] == '+' || expr[i][0] == '-' || expr[i][0] == '*' || expr[i][0] == '/') {
while (!st.empty()) {
op = (st.top()[0]);
if (pre(expr[i][0]) <= pre(op[0]))
postfix.push_back(st.pop());
else break;
}
postfix.push_back(expr[i]);
}
else if (expr[i][0] == '(') {
st.push(expr[i]);
}
else if (expr[i][0] == ')') {
while (!st.empty()) {
op = st.pop();
if (op[0] == '(') break;
else {
postfix.push_back(op);
}
}
}
else {
postfix.push_back(expr[i]);
}
}
return postfix;
}
int main() {
auto expr = infix_to_postfix({ "(", "2", "+", "3", ")", "*", "7" });
for (auto& elem : expr)
{
std::cout << elem << ", ";
}
std::cout << "n";
return 0;
}

这个程序应该从中缀转换为后缀，但它没有产生正确的输出。我希望输出是:

2, 3, +, 7, *,

但是输出是

2, +, 3, *, 7,

Answer 1

你有两个问题

1. 处理操作符时，在弹出低优先级操作符后，需要将新操作符压入堆栈，而不是将其添加到结果中:

if (expr[i][0] == '+' || expr[i][0] == '-' || expr[i][0] == '*' || expr[i][0] == '/') {
while (!st.empty()) {
op = (st.top()[0]);
if (pre(expr[i][0]) <= pre(op[0]))
postfix.push_back(st.pop());
else break;
}
st.push(expr[i]);
}

函数的最后你需要流行任何剩余的运营商从堆栈中:

while (!st.empty())
{
postfix.push_back(st.pop());
}
return postfix;