LeetCode: 150_Evaluate Reverse Polish Notation | 分析逆波兰式 | Medium

Evaluatethe 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

int evalRPN(vector<string> &tokens)
{
stack<int> stokens;
for (int i = 0; i < tokens.size(); ++ i) {
if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
int x1 = stokens.top();
stokens.pop();
int x2 = stokens.top();
stokens.pop();
int x3 = caculate(x1, x2, tokens[i]); //计算之后再入栈
stokens.push(x3);
}
else {
stokens.push(atoi(tokens[i].c_str()));
}
}
return stokens.top();
}

int caculate(int x1, int x2, string s)
{
int result;
if (s == "+")
result = x1 + x2;
else if (s == "-")
result = x1 - x2;
else if (s == "*")
result = x1 * x2;
else {
if (x1 >= x2)
result = x1 / x2;
else
result = x2 / x1;
}
return result;
}