title: Calculator
date: 2022-04-18 15:15:13
tags: Make a little progress every day

Daily topic

subject ： Calculator

Given an integer containing a positive integer 、 Add (+)、 reduce (-)、 ride (*)、 except (/) Arithmetic expression of ( Except brackets ), Calculate the result .

The expression contains only non negative integers ,+, - ,*,/ Four operators and spaces . Division of integers preserves only the integral part .

Example ：

Example 1:

Input : “3+2*2”
Output : 7

Example 2:

Input : " 3/2 "
Output : 1

Example 3:

Input : " 3+5 / 2 "
Output : 5

explain ：

You can assume that the given expressions are valid .
Please do not use built-in library functions eval.

Small pit , There are spaces in the string , Need to replace

Code ：

class Solution {
    
    public int calculate(String s) {
    
        s =s.replaceAll(" ","");
        Deque<Integer> stack = new ArrayDeque<>();
        // First add the first one 
        char flag = '+';
        // Initial value of the number 
        int num = 0;
        for(int i=0;i<s.length();i++){
    
            // Convert to numbers 
            if(Character.isDigit(s.charAt(i))){
    
                num = num*10+s.charAt(i)-'0';
            }
            // Run according to the symbol , Namely * and /, High priority , First take out the numbers in the stack , Put it in after calculation 
            if(!Character.isDigit(s.charAt(i)) || i==s.length()-1){
    
                if(flag=='+'){
    
                    stack.push(num);
                }else if(flag=='-'){
    
                    stack.push(-num);
                }else if(flag=='*'){
    
                    stack.push(stack.pop()*num);
                }else if(flag=='/'){
    
                    stack.push(stack.pop()/num);
                }
                flag = s.charAt(i);
                num =0;
            }

        }
        // Traversal stack , Cumulative results 
        int res =0;
        while(!stack.isEmpty()){
    
            res += stack.pop();
        }

        return res;


    }
}

That's all about the calculator

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Original Blogger ： Cowherd Conan
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