Eric 的小站

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输入：tokens = ["2","1","+","3","*"]
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输出：9
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解释：该算式转化为常见的中缀算术表达式为：((2 + 1) * 3) = 9

1
输入：tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
2
输出：22
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解释：该算式转化为常见的中缀算术表达式为：
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  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
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= ((10 * (6 / (12 * -11))) + 17) + 5
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= ((10 * (6 / -132)) + 17) + 5
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= ((10 * 0) + 17) + 5
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= (0 + 17) + 5
9
= 17 + 5
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= 22

1
class Solution {
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public:
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    bool isNumber(const string& str) {
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        if (str.empty())
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            return false;
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        int start = (str[0] == '-') ? 1 : 0; // 检查负数
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        if (start == 1 && str.size() == 1) // 只有一个负号的情况
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            return false;
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        for (int i = start; i < str.size(); i++) {
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            if (!isdigit(str[i]))
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                return false;
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        }
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        return true;
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    }
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    int evalRPN(vector<string>& tokens) {
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        stack<int> stk;
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        for (auto token : tokens) {
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            if (isNumber(token)) {
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                stk.push(stoi(token));
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            } else {
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                int b = stk.top();
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                stk.pop();
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                int a = stk.top();
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                stk.pop();
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                if (token.compare("+") == 0) {
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                    stk.push(a + b);
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                } else if (token.compare("-") == 0) {
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                    stk.push(a - b);
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                } else if (token.compare("*") == 0) {
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                    stk.push(a * b);
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                } else if (token.compare("/") == 0) {
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                    stk.push(a / b);
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                }
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            }
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        }
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        return stk.top();
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    }
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};