Evaluate Reverse Polish Notation (STRING-TYPE CONVERTION)

Question

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

1ST TRY

class Solution {
public:
int evalRPN(vector<string> &tokens) {
int operant1;
int operant2;
string str;
int ret;
bool firstOp = false;

for(int i = 0; i < tokens.size(); i++)
{
str = tokens[i];
if(!firstOp)
{
operant1 = getInt(str);;
firstOp = true;
}
else if(str == "+")
{
ret = operant1 + operant2;
}
else if(str == "-")
{
ret = operant1 - operant2;
}
else if(str == "*")
{
ret = operant1 * operant2;
}
else if(str == "/")
{
ret = operant1 / operant2;
}
else
{
operant2 = getInt(str);
firstOp = false;
}
}

return ret;
}
int getInt(string str)
{
int ret = 0;
for(int i = 0; i < str.length(); i++)
{
ret = ret*10 + str[i];
}
return ret;
}
};

2ND TRY

考虑只有一个操作数的情况

class Solution {
public:
int evalRPN(vector<string> &tokens) {
int operant1;
int operant2;
string str;
int ret;
bool firstOp = false;

for(int i = 0; i < tokens.size(); i++)
{
str = tokens[i];
if(!firstOp)
{
operant1 = getInt(str);
firstOp = true;
}
else if(str == "+")
{
operant1 += operant2;
}
else if(str == "-")
{
operant1 -= operant2;
}
else if(str == "*")
{
operant1 *= operant2;
}
else if(str == "/")
{
operant1 /= operant2;
}
else
{
operant2 = getInt(str);
}
}
return operant1;
}

int getInt(string str)
{
int ret = 0;
for(int i = 0; i < str.length(); i++)
{
ret = ret*10 + (str[i]-'0');
}
return ret;
}
};

Result: Wrong

Input: ["3","-4","+"]
Output: -23
Expected: -1

3RD TRY

考虑负数的情况

class Solution {
public:
int evalRPN(vector<string> &tokens) {
int operant1;
int operant2;
int ret;
string str;
stack<int> operantStack;

for(int i = 0; i < tokens.size(); i++)
{
str = tokens[i];
if(str == "+")
{
operant2 = operantStack.top();
operantStack.pop();
operant1 = operantStack.top();
operantStack.pop();
operantStack.push(operant1 + operant2);
}
else if(str == "-")
{
operant2 = operantStack.top();
operantStack.pop();
operant1 = operantStack.top();
operantStack.pop();
operantStack.push(operant1 - operant2);
}
else if(str == "*")
{
operant2 = operantStack.top();
operantStack.pop();
operant1 = operantStack.top();
operantStack.pop();
operantStack.push(operant1 * operant2);
}
else if(str == "/")
{
operant2 = operantStack.top();
operantStack.pop();
operant1 = operantStack.top();
operantStack.pop();
operantStack.push(operant1 / operant2);
}
else
{
operantStack.push(getInt(str));
}
}
return operantStack.top();
}

int getInt(string str)
{
int ret = 0;
bool negFlag = false;
for(int i = 0; i < str.length(); i++)
{
if(str[i]=='-') negFlag = true;
else if(negFlag)
{
ret = ret*10 - (str[i]-'0');
}
else
{
ret = ret*10 + (str[i]-'0');
}
}
return ret;
}
};

Result: Accepted

4TH TRY

使用内置函数atoi将string转换成int

class Solution {
public:
int evalRPN(vector< string > &tokens) {
stack< int > operandStack;
int operand1;
int operand2;
for(int i = 0; i < tokens.size(); i++){
if(tokens[i]=="+"){
operand1 = operandStack.top();
operandStack.pop();
operand2 = operandStack.top();
operandStack.pop();
operand2 += operand1;
operandStack.push(operand2);
}
else if(tokens[i]=="-"){
operand1 = operandStack.top();
operandStack.pop();
operand2 = operandStack.top();
operandStack.pop();
operand2 -= operand1;
operandStack.push(operand2);
}
else if(tokens[i]=="*"){
operand1 = operandStack.top();
operandStack.pop();
operand2 = operandStack.top();
operandStack.pop();
operand2 *= operand1;
operandStack.push(operand2);
}
else if(tokens[i]=="/"){
operand1 = operandStack.top();
operandStack.pop();
operand2 = operandStack.top();
operandStack.pop();
operand2 /= operand1;
operandStack.push(operand2);
}
else{
operand1 = atoi(tokens[i].c_str());
operandStack.push(operand1);
}
}
return operandStack.top();
}
};

Result: Accepted