面试题77：前缀、中缀、后缀表达式的相互转换

#include<iostream>
#include <vector>
#include <stack>
using namespace std;

bool IsOperator(char c)
{
const char ops[] = "+-*/";
for (int i = 0; i < sizeof(ops); i++)
{
if (c == ops[i]) return true;
}
return false;
}

/*
判断优先级：
如果a比b高，返回1,；如果相等，返回0；如果a比b低，返回-1.
*/
int Firster(char a, char b)
{
if (a == '(') return -1;   //优先级最低
if (a == '+' || a=='-')
{
if (b == '*' || b == '/') return -1;
else return 0;
}
if (a == '*' || a == '/')
{
if (b == '+' || b == '-') return 1;
else return 0;
}
return 0;  //这句只是为了消除警告
}

/*
将前缀转为后缀
*/
void InfixToSuffix(const vector<char> infix, vector<char> &suffix)
{
stack<char> oChar;
int size = infix.size();
char temp;
//int index = 0;
for (int i = 0; i < size; i++)
{
temp = infix[i];
if (temp == '(') oChar.push(temp);
else if (temp == ')')
{
while (oChar.top() != '(')
{
suffix.push_back(oChar.top());
oChar.pop();
}
oChar.pop();
}
else
{
if (!IsOperator(temp)) oChar.push(temp);
else
{
while (!oChar.empty() && Firster(oChar.top(), temp) >= 0) //理解这部分
{
suffix.push_back(oChar.top());
oChar.pop();
}
oChar.push(temp);
}

}
}
while (!oChar.empty())
{
suffix.push_back(oChar.top());
oChar.pop();
}
suffix.push_back('\0');
}

int main(){
char arr[] = "9+(3-1)*3+9/2";
vector<char> infix(arr, arr + strlen(arr));
vector<char> re;
InfixToSuffix(infix, re);
for (unsigned int i = 0; i < re.size(); i++)
cout << re[i];
cout << endl;
return 0;
}