LintCode_424_逆波兰表达式求值

问题描述

样例

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

求解

class Solution {
public:
/*
* @param tokens: The Reverse Polish Notation
* @return: the value
*/
int evalRPN(vector<string> tokens) {
if (tokens.empty()) {
return 0;
}
stack<int> s;

auto is_operator = [](const string &op) -> bool {
return op.length() == 1 && string("+-*/").find(op) != string::npos;
};

for (const auto& tok : tokens) {
if (!is_operator(tok)) {
s.emplace(stoi(tok));
} else {
int y = s.top();
s.pop();
int x = s.top();
s.pop();
if (tok[0] == '+') {
x += y;
} else if (tok[0] == '-') {
x -= y;
} else if (tok[0] == '*') {
x *= y;
} else {
x /= y;
}
s.emplace(x);
}
}
return s.top();
}
};