leetcode150 Evaluate Reverse Polish Notation

 1 """
 2 Evaluate the value of an arithmetic expression in Reverse Polish Notation.
 3 Valid operators are +, -, *, /. Each operand may be an integer or another expression.
 4 Note:
 5     Division between two integers should truncate toward zero.
 6     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.
 7 Example 1:
 8 Input: ["2", "1", "+", "3", "*"]
 9 Output: 9
10 Explanation: ((2 + 1) * 3) = 9
11 Example 2:
12 Input: ["4", "13", "5", "/", "+"]
13 Output: 6
14 Explanation: (4 + (13 / 5)) = 6
15 Example 3:
16 Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
17 Output: 22
18 Explanation:
19   ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
20 = ((10 * (6 / (12 * -11))) + 17) + 5
21 = ((10 * (6 / -132)) + 17) + 5
22 = ((10 * 0) + 17) + 5
23 = (0 + 17) + 5
24 = 17 + 5
25 = 22
26 """
27 """
30The first is arithmetic operations character into integer To
29This question is very simple ideas can be realized with a stack own AC, but encountered trouble three
28   The second is the minuend and subtraction problems. The first pop-up is subtractive, pop-up is the minuend. Also need to consider this division
 31  and the third is a problem python rounded down 132 // == -1 -6
 32  "" " 
33 is  class Solution1:
 34 is      DEF evalRPN (Self, tokens):
 35          IF  Not tokens:
 36              return 0
 37 [          Stack = []
 38 is          for I in Range (len (tokens)):
 39              stack.append (tokens [I])
 40              IF tokens [I] == ' + ' :
 41 is                  stack.pop ()
 42 is                  X = int (stack.pop ())
43 is                  Y = int (stack.pop ())
 44 is                  stack.append (Y + X)
 45              IF tokens [I] == ' - ' :
 46 is                  stack.pop ()
 47                  X = int (stack.pop ())
 48                  Y = int (stack.pop ())
 49                  stack.append (YX) # subtractor Note that the first out stack subtraction, division with 
50              IF tokens [I] == ' * ' :
 51 is                  stack.pop ()
 52 is                  X = int (stack.pop ())
 53                 = Y int (stack.pop ())
 54 is                  stack.append (Y * X)
 55              IF tokens [I] == ' / ' :
 56 is                  stack.pop ()
 57 is                  X = int (stack.pop ())
 58                  Y = int (stack.pop ())
 59                  IF X> 0 and Y <0 or X <0 and Y> 0:
 60                      stack.append (- (ABS (Y) // ABS (X)))
 61 is                  the else :                     # 6 // - 132 == -1, python rounded down, so the addition of this statement
62                     stack.append(y//x)
63         return stack.pop()
64 
65 if __name__ == '__main__':
66     ans = Solution1()
67     x = ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
68     s = ans.evalRPN(x)
69     print(s)
70 
71 print(6//-132)