Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are
+
, -
, *
, /
. Each operand may be an integer or another expression.
Note:
- Division between two integers should truncate toward zero.
- The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
Example 1:
Input: ["2", "1", "+", "3", "*"] Output: 9 Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: ["4", "13", "5", "/", "+"] Output: 6 Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"] Output: 22 Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5 = ((10 * (6 / (12 * -11))) + 17) + 5 = ((10 * (6 / -132)) + 17) + 5 = ((10 * 0) + 17) + 5 = (0 + 17) + 5 = 17 + 5 = 22
A:
每遇到运算符,就提出最后面的两个,运算,并保存。class Solution { public: int evalRPN(vector<string>& tokens) { stack<int> mystack; for(auto s:tokens) { if(s == "+" || s == "-" || s == "*" || s == "/") { int a = mystack.top(); mystack.pop(); int b = mystack.top(); mystack.pop(); if(s == "+") mystack.push(b+a); if(s == "-") mystack.push(b-a); if(s == "*") mystack.push(b*a); if(s == "/") mystack.push(b/a); }else{ mystack.push(stoi(s)); } } return mystack.top(); } };
Mistakes:
1 : 没有考虑 当一个数为:负数的时候, 例如 -2, 如果只检查首字母, 就会误认为是运算符。
2: 刚开始,自以为是了。 用double类型保持中间值。
没想到,作者是不用的。 用int数组就行。
----------第二遍-----------
3: 由于是从 + 的情况直接copy 的代码, 对于 - / 的情况,没有考虑 a,b 的顺序要调过来的。
Learned:
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