LeetCode-150-如何评估逆波兰表达式？

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算法描述：计算逆波兰表达式（Reverse Polish Notation，RPN）的值。有效的运算符包括：+、-、*、/。每个操作数可以是整数或另一个表达式。注意：两个整数之间的除法应向零取整。

pythondef evaluate_rpn(expression): stack=[] for token in expression.split(): if token.isdigit() or (token[0]=='-' and token[1:].isdigit()): stack.append(int(token)) else: b=stack.pop() a=stack.pop() if token=='+': stack.append(a + b) elif token=='-': stack.append(a - b) elif token=='*': stack.append(a * b) elif token=='/': stack.append(int(a / b)) return stack[0]

算法描述：

Evaluate the value of an arithmetic expression inReverse 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‘tbe anydivideby 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

解题思路：逆波特兰表达式，用栈辅助模拟。

int evalRPN(vector<string>& tokens) { if(tokens.size()==0) return 0; stack<int> stk; for(auto c : tokens){ if(c == "+" || c == "-" || c== "*" || c=="/"){ int right = stk.top(); stk.pop(); int left = stk.top(); stk.pop(); int ans; if(c == "+") ans = left + right; if(c == "-") ans = left - right; if(c == "*") ans = left * right; if(c == "/") ans = left / right; stk.push(ans); }else{ stk.push(stoi(c)); } } return stk.top(); }