前缀、后缀表达式求值c++代码

前缀表达式求值

​
#include<bits/stdc++.h>
using namespace std;
string s;
int main(){
	int i;
	stack<int> q;
	cout<<"Please enter a prefix expression\n";
	getline(cin,s);
	for(i=s.size()-1;i>=0;i--){
		if(s[i]>='0'&&s[i]<='9'){
			if(s[i-1]<'0'||s[i-1]>'9'){
				int x=s[i]-'0';
				q.push(x);
			}
			else{
				int f=s[i]-'0';
				i--;
				while(s[i]>='0'&&s[i]<='9'){
					f*=10;
					f+=s[i]-'0';
					i--;
				}
				string s1=to_string(f);
				reverse(s1.begin(),s1.end());
				int x=stoi(s1);
				q.push(x);
			}
		}
		else{
			if(s[i]=='+'||s[i]=='-'||s[i]=='*'||s[i]=='/'){
				int t1=q.top();
				q.pop();
				int t2=q.top();
				q.pop();
				if(s[i]=='+'){
					int x=t1+t2;
					q.push(x);
				}
				else{
					if(s[i]=='-'){
						int x=t1-t2;
						q.push(x);
					}	
					else{
						if(s[i]=='*'){
							int x=t1*t2;
							q.push(x);
						}	
						else{
							int x=t1/t2;
							q.push(x);
						}	
					}
				}
			}	
		}
	}	
	cout<<"The value is:"<<q.top();
	return 0;
}

​

后缀表达式求值

#include<bits/stdc++.h>
using namespace std;
string s;
int main(){
	int i;
	stack<int> q;
	cout<<"Please enter a suffix expression\n";
	getline(cin,s);
	int n=s.size()-1;
	for(i=0;i<=n;i++){
		if(s[i]>='0'&&s[i]<='9'){
			if(s[i+1]<'0'||s[i+1]>'9'){
				int x=s[i]-'0';
				q.push(x);
			}
			else{
				int f=s[i]-'0';
				i++;
				while(s[i]>='0'&&s[i]<='9'){
					f*=10;
					f+=s[i]-'0';
					i++;
				}
				string s1=to_string(f);
				reverse(s1.begin(),s1.end());
				int x=stoi(s1);
				q.push(x);
			}
		}
		else{
			if(s[i]=='+'||s[i]=='-'||s[i]=='*'||s[i]=='/'){
				int t1=q.top();
				q.pop();
				int t2=q.top();
				q.pop();
				if(s[i]=='+'){
					int x=t2+t1;
					q.push(x);
				}
				else{
					if(s[i]=='-'){
						int x=t2-t1;
						q.push(x);
					}	
					else{
						if(s[i]=='*'){
							int x=t2*t1;
							q.push(x);
						}	
						else{
							int x=t2/t1;
							q.push(x);
						}	
					}
				}
			}	
		}
	}		
	cout<<"The value is:"<<q.top();
	return 0;
}