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
原题链接: https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/
题目大意: 逆波兰表达式
思路: 利用栈实现, 注意+, -单目运算符; 另外目前leetcode使用的python为2.7.5, 除法不是精确的除法, 两个整数相除是地板除, 浮点数除是精确的除法; 注意负数的判断, 使用isdigit是判断不了的;
1 class Solution: 2 # @param tokens, a list of string 3 # @return an integer 4 def evalRPN(self, tokens): 5 stack = list() 6 7 for token in tokens: 8 try: 9 tmp = int(token) 10 except ValueError: #operator 11 if token == '+': #may be a unary operator 12 if len(stack) > 1: 13 o1 = stack.pop() 14 o2 = stack.pop() 15 16 stack.append(o2 + o1) 17 elif token == '-': 18 if len(stack) > 1: 19 o1 = stack.pop() 20 o2 = stack.pop() 21 22 stack.append(o2 - o1) 23 else: 24 o1 = stack.pop() 25 stack.append(-o1) 26 27 elif token == '*': 28 o1 = stack.pop() 29 o2 = stack.pop() 30 31 stack.append(o2 * o1) 32 else: 33 o1 = stack.pop() 34 o2 = stack.pop() 35 36 stack.append(int(float(o2) / o1)) 37 38 else: #number 39 stack.append(tmp) 40 41 42 return stack[0]