poj 1686 表达式求值判断

这题考查两个点

1、先将算术表达式转换成后缀表达式，转换原理可以参考这个博客，写得非常详细http://blog.youkuaiyun.com/antineutrino/article/details/6763722

2、再对后缀表达式这里算值。

小心：

这里输入必须要整行读入，因为表达式中可能会揉进一些空格和tab，所以使用cin读入会出错。

另外，混合使用cin和cin.getline时，使用cin之后，会在缓冲区留下一个换行，会被下面的cin.getline吃掉，所以第一个cin.getline无法读到正确的数据，必须中间执行一次cin.ignore()，清空缓冲区。

代码如下：

//by werflychen 2013-12-21
#include <iostream>
#include <string>
#include <stack>

using namespace std;

inline bool IsLegal(char ch)
{
	if (ch == '+' || ch == '-' || ch == '*' || ch == '(' || ch == ')'
		|| (ch >= '0' && ch <= '9') || (ch >= 'a' && ch <= 'z')
		|| (ch >= 'A' && ch <= 'Z'))
	{
		return true;
	}
	return false;
}

void change(char *inorder, char *postorder)
{
	if (inorder == NULL)
	{
		return;
	}

	stack<char> sta_operator;
	int j = 0;

	char *p = inorder;
	while('\0' != *p)
	{
		if (IsLegal(*p))
		{
			switch(*p)
			{
				case '+': 
				case '-': 
					{
						//遇到sta_operator栈顶运算符相同或较高的情况下，将栈顶运算符先出栈压入postorder，再将自身入栈
						if (!sta_operator.empty() && ('+' == sta_operator.top() || '-' == sta_operator.top() || '*' == sta_operator.top()))
						{
							char chtmp = sta_operator.top();
							postorder[j++] = chtmp;
							sta_operator.pop();
							sta_operator.push(*p);
						}
						//其他情况都直接进sta_operator
						else
						{
							sta_operator.push(*p);
						}
						break;
					}
				case '*': 
					{
						if (!sta_operator.empty() && '*' == sta_operator.top())
						{
							char chtmp = sta_operator.top();
							postorder[j++] = chtmp;
							sta_operator.pop();
							sta_operator.push(*p);
						}
						else
						{
							sta_operator.push(*p);
						}
					
						break;
					}
				case '(':
					{
						sta_operator.push(*p);
						break;
					}
				
				case ')':
					//将sta_operator栈顶开始弹栈，直至遇到'('
					{
						char ch = sta_operator.top();
						bool flag = sta_operator.empty();
						while (!sta_operator.empty())
						{
							if ('(' == sta_operator.top())
							{
								sta_operator.pop();
								break;
							}
							else
							{
								char chtmp = sta_operator.top();
								postorder[j++] = chtmp;
								sta_operator.pop();
							}
						}
					}
					break;
				
				default:
					postorder[j++] = *p;
					break;
			}
		}
		++p;
	}
	
	if (!sta_operator.empty())
	{
		postorder[j++] = sta_operator.top();
	}
}

inline int CAL(char chOperator, int n1, int n2)
{
	int iRes = 0;
	switch (chOperator)
	{
		case '+':
			{
				iRes = n1 + n2;
				break;
			}
		case '-':
			{
				iRes = n1 - n2;
				break;
			}
		case '*':
			{
				iRes =  n1 * n2;
				break;
			}
	}
	
	return iRes;
}



bool CalResult(char *strPost, int &iRes)
{
	if (NULL == strPost)
	{
		return false;
	}

	stack<int> sta_operator;
	char *p = strPost;
	
	while(*p != '\0')
	{
		switch (*p)
		{
			case '+':
			case '-':
			case '*':
				{
					if (!sta_operator.empty())
					{
						int n2 = sta_operator.top();
						sta_operator.pop();
						int n1 = 0;
						if (!sta_operator.empty())
						{
							n1= sta_operator.top();
							sta_operator.pop();
						}
						else
						{
							return false;
						}
						int iCurRes = CAL(*p, n1, n2);
						sta_operator.push(iCurRes);
					}
					else
					{
						return false;
					}
					break;
				}
			default:
				{
					if (*p >= '0' && *p <= '9')
					{
						sta_operator.push((int)(*p - '0'));
					}
					else
					{
						sta_operator.push((int)*p);
					}
					break;
				}
		}

		++p;
	}

	iRes = sta_operator.top();
	return true;
}


int main()
{
//	freopen("input.txt", "r", stdin);
	char str1[100];
	char strPost1[100];
	char str2[100];
	char strPost2[100];
	
	int iRes1 = 0, iRes2 = 0;
	int num = 0;
	cin >> num;
	cin.ignore();
	for (int i = 0; i < num; ++i)
	{

		cin.getline(str1, 100, '\n');
		cin.getline(str2, 100, '\n');
	
		memset((void*)strPost1, 0, sizeof(strPost1));
		memset((void*)strPost2, 0, sizeof(strPost2));
		change(str1, strPost1);
		change(str2, strPost2);
// 			cout << "strPost1 = " << strPost1 << endl;
// 			cout << "strPost2 = " << strPost2 << endl;
		int b1 = CalResult(strPost1, iRes1);
		int b2 = CalResult(strPost2, iRes2);
		if (iRes1 == iRes2)
		{
			cout << "YES" << endl;
		}
		else
		{
			cout << "NO" << endl;
		}
// 			cout << "iRes1 = " << iRes1 << " b1 = " << b1 << endl;
// 			cout << "iRes2 = " << iRes2 << " b2 = " << b2 << endl;
	
	}

	//cout << endl;

	return 0; 
}