文章标题

[leetcode]evaluate-reverse-polish-notation

题目内容

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

分析

题目为分析逆波兰表达式，然后得出结果。即算术表达式的求值过程。题目要求为int类型运算和简单的+-*/四则运算。在除法时需要考虑被除数为0的情况。

int evalRPN(vector<string> &tokens) {
std::stack<int> stackNum;
if(tokens.size() == 0)
{
return  0;
}
int a,b;
while( !tokens.empty() )
{
string str = tokens.front();
tokens.erase(tokens.begin());
if(str == "+")
{
a = stackNum.top();
stackNum.pop();
b = stackNum.top();
stackNum.pop();
stackNum.push(a+b);
}
else if(str == "-")
{
a = stackNum.top();
stackNum.pop();
b = stackNum.top();
stackNum.pop();
stackNum.push(b-a);
}
else if(str == "*")
{
a = stackNum.top();
stackNum.pop();
b = stackNum.top();
stackNum.pop();
stackNum.push(a*b);
}
else if(str == "/")
{
a = stackNum.top();
stackNum.pop();
b = stackNum.top();
stackNum.pop();
if(a != 0)
stackNum.push(b/a);
}
else
{
int num = atoi(str.c_str());
stackNum.push(num);
}
}
return stackNum.top();
}