//postfix expression test whether the postfix expression is right
#include<string>#include<iostream>using namespace std;int postfix(const string &str,int cur,int size);int main(){string str;cout<<"please the computing expression :";cin>>str;int size=str.size();int index=postfix(str,size-1,size);cout<<str<<endl;if(index>-1&&index==0)cout<<"the postifix expression is right !"<<endl;else cout<<"the postfix expression is wrong !"<<endl;return 0;}int postfix(const string &str,int cur,int size)//find the first of postfix,if one exists//precondition :the substring of str from the index cur//through the first of the string contains no blanks character//postcondition :returns the index of the first character//int the postfix expression that begins at index end of str//firstPtr should rturns -1 if no such prefix expression exists{if(cur<0||cur>=size)return -1;if(str[cur]>='a'&&str[cur]<='z')return cur;else if(str[cur]=='*'||str[cur]=='+'||str[cur]=='-'||str[cur]=='/'){int pre=postfix(str,cur-1,size);if(pre>-1)return postfix(str,pre-1,size);elsereturn -1;}elsereturn -1;}bool infix(const string &str,int first,int last){if((first==last)&&(str[first]>='a')&&(str[first]<='z'))return true;else{int mid=findO(str,first,last);if(mid==-1)return false;else{bool ok=infix(str,first,mid-1);if(ok)return infix(str,mid+1,last);elsereturn false;}}}//prefix expression test whether the prefix expression is right
#include<string>#include<iostream>using namespace std;int prefix(const string &str,int cur,int size);int main(){string str;cout<<"please enter prefix expression :";cin>>str;int size=str.size();int index=prefix(str,0,size);cout<<str<<endl;if(index>-1&&index==size-1)cout<<"prefix expression success !"<<endl;elsecout<<"prefix expression failed !"<<endl;}int prefix(const string &str,int cur,int size)//Finds the end of a prefix expression ,if one exists.//precondition :the substring of str from index cur//through the end of the string contains no blank characters//Postcondition :return the index of the lase character//int the prefix expression that begins at index first of //strExp endPtr should returns -1 if no such prefix expression exist{if(cur<0||cur>=size)return -1;if(str[cur]>='a'&&str[cur]<='z')return cur;else if(str[cur]=='*'||str[cur]=='+'||str[cur]=='-'||str[cur]=='/'){int post=prefix(str,cur+1,size);if(post>-1)return prefix(str,post+1,size);elsereturn -1;}elsereturn -1;}
//and turn the postfix expression to the prefix expression
#include<iostream>#include<string>using namespace std;void pre2post(const string &str,string &post,int &index);int main(){cout<<"please enter a right prefix expression :";string str,post;cin>>str;cout<<endl<<str<<endl;int size=0;pre2post(str,post,size);cout<<" covert to postfix expression :"<<post<<endl;return 0;}void pre2post(const string &str,string &post,int &index){if(index<str.size()){ char ch=str[index];index++; if(ch>='a'&&ch<='z') post=post+ch; else { pre2post(str,post,index); pre2post(str,post,index); post=post+ch; } }}
