[LeetCode] Evaluate Reverse Polish Notation

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

Solution:

需要注意operand为负数的情况。

class Solution {
public:
int stringToInt(string s)
{
if(s.length() == 0)
return 0;
int p = 1;
if(s[0] == '-')
{
p = -1;
s = s.substr(1, s.length() - 1);
}
int ans = s[0] - '0';
for(int i = 1;i < s.length();i++)
ans = ans * 10 + s[i] - '0';
return ans * p;
}

int evalRPN(vector<string> &tokens) {
int len = tokens.size(), ans = 0;
int *operand = new int[len + 1], operandNum = 0;
memset(operand, 0, (len + 1) * sizeof(int));

int res = 0; // 0 for num, 1 +, 2 -, 3 *, 4 /
for(int i = 0;i < len;i++)
{
if(tokens[i] == "+")
{
operand[operandNum - 2] = operand[operandNum - 2] + operand[operandNum - 1];
operandNum--;
}
else if(tokens[i] == "-")
{
operand[operandNum - 2] = operand[operandNum - 2] - operand[operandNum - 1];
operandNum--;
}
else if(tokens[i] == "*")
{
operand[operandNum - 2] = operand[operandNum - 2] * operand[operandNum - 1];
operandNum--;
}
else if(tokens[i] == "/")
{
operand[operandNum - 2] = operand[operandNum - 2] / operand[operandNum - 1];
operandNum--;
}
else
{
int op = stringToInt(tokens[i]);
operand[operandNum++] = op;
}
}
return operand[0];
}
};