227. Basic Calculator

1. Description of the problem

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +,-, *,/operators and empty spaces. The integer division should truncate toward zero.

Assume that the given expression was always valid.

Some Examples:

" 3+2*2 " 7 "  "1""5

Note: Do not use the Eval built-in library function.

Tags: String

Similar Problems: H Basic Calculator, (H) Expression Add Operators

Solution:

class Solution {public:    int calculate (string  s) {    }};  

2. Answer your ideas2.1 Legitimacy Judgments

• Empty string
• Before the first numeric character (' 0 ' ~ ' 9 ') appears (assuming at position I), then the character of position 0 to position I can only be a space ' or a sign ' + ' or '-'
• Except for numbers, plus, minus, multiply, divide, and space characters, all other characters are illegal characters, returning 0. At the same time, you need to set the global flag bit, and the calculation result is 0 to distinguish
• Is it possible to have multiple operator characters adjacent? e.g. "-+/", "*/-" etc. No, because the title says you could assume that the given expression was always valid.
• If there is overflow how to deal with

2.2 Integer Extraction

• How to extract an integer from a string that is adjacent to multiple spaces with numeric characters

2.3 Algorithm

3. Code

Hair

4. Reflection

Hair

227. Basic Calculator