表达式求值（逆波兰）

博主CSDN

#include<stdio.h>
#include<bits/stdc++.h>
using namespace std;
const int maxn = 1e3 + 10;
typedef long long ll;
char s[maxn];
int cmp(char ch){
	switch (ch){
		case '+':
		case '-':return 1;
		case '*':
		case '/':return 2;
		default:return 0;
	}
}
string ss = "";
void ToSuffix(){
	stack<char>Se;
	double ans = 0;
	Se.push('#');
	int len = strlen(s);
	for (int i = 0; i<len; i++){
		if (s[i] >= '0'&&s[i] <= '9'){
			ss += s[i++];
			while (s[i] >= '0'&&s[i] <= '9'){
				ss += s[i++];
			}
			ss += ' ';
		}
		if (s[i] == '(')Se.push(s[i]);
		else if (s[i] == ')'){
			while (Se.top() != '('){
				ss += Se.top();
				ss += ' ';
				Se.pop();
			}
			Se.pop();
		}
		else if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/'){
			while (cmp(Se.top()) >= cmp(s[i])){
				ss += Se.top();
				ss += ' ';
				Se.pop();
			}
			Se.push(s[i]);
		}
	}
	while (Se.top() != '#'){
		ss += Se.top();
		ss += ' ';
		Se.pop();
	}
	Se.pop();
}
double Solve(){
	stack<double>num;
	int i = 0;
	double x = 0, y = 0,ans=0;
	for (int i = 0; ss[i]; i++){
		if (ss[i] >= '0'&&ss[i] <= '9'){
			ans = ss[i++] - '0';
			while (ss[i] >= '0'&&ss[i] <= '9'){
				ans =ans*10+ ss[i++]-'0';
			}
			num.push(ans);
			ans = 0;
		}
		else if (ss[i] == ' ');
		else{
			x = num.top();
			num.pop();
			y = num.top();
			num.pop();
			if (ss[i] == '+')num.push(x + y);
			else if (ss[i] == '-')num.push(y - x);
			else if (ss[i] == '*')num.push(x * y);
			else if (ss[i] == '/')num.push(y / x);
		}
	}
	x = num.top();
	num.pop();
	return x;
}
int main(){
	freopen("C:/Users/Administrator/Desktop/input.txt", "r", stdin);
	int t;
	scanf("%d", &t);
	while (t--){
		scanf("%s", s);
		ss = "";
		ToSuffix();
		printf("%.2f\n",Solve());
	}
	return 0;
}