LeetCode150：Evaluate Reverse Polish Notation

题目：

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) –> 6

解题思路：

很简单的一题，直接利用栈实现，不多说了

实现代码：

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

/*
Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
*/
class Solution {
public:
int evalRPN(vector<string> &tokens) {
stack<string> operand_stack;
vector<string>::iterator iter;
for(iter = tokens.begin(); iter != tokens.end(); ++iter)
{
if(*iter == "+")//这里就不提取重复代码了，直接简单点
{
string op1 = operand_stack.top();
operand_stack.pop();
string op2 = operand_stack.top();
operand_stack.pop();
//int ret = atoi(op2.c_str()) + atoi(op1.c_str());
int ret = stoi(op2) + stoi(op1);//stoi为C++11才有
string result = to_string(ret);
operand_stack.push(result);

}
else if(*iter == "-")
{
string op1 = operand_stack.top();
operand_stack.pop();
string op2 = operand_stack.top();
operand_stack.pop();
int ret = atoi(op2.c_str()) - atoi(op1.c_str());
string result = to_string(ret);
operand_stack.push(result);

}
else if(*iter == "*")
{
string op1 = operand_stack.top();
operand_stack.pop();
string op2 = operand_stack.top();
operand_stack.pop();
int ret = atoi(op2.c_str()) * atoi(op1.c_str());
string result = to_string(ret);
operand_stack.push(result);

}
else if(*iter == "/")
{
string op1 = operand_stack.top();
operand_stack.pop();
string op2 = operand_stack.top();
operand_stack.pop();
if( atoi(op1.c_str()) == 0)
return 0x7FFFFFF;
int ret = atoi(op2.c_str()) / atoi(op1.c_str());
string result = to_string(ret);
operand_stack.push(result);

}
else
operand_stack.push(*iter);
}
return atoi(operand_stack.top().c_str());

}

};

int main(void)
{
string strs[] = {"4", "13", "5", "/", "+"};
vector<string> tokens(strs, strs+5);
Solution solution;
int ret = solution.evalRPN(tokens);
cout<<ret<<endl;
return 0;
}