# Leetcode 227. Basic Calculator II Java language

Source: Internet
Author: User
Tags integer division

`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 "3/2" = 1 "3+5/2" = 5note:do not use the Eval built-in library function.`

Test instructions: Evaluation

`Public class solution {    public int calculate (String s)  {        int digt=0;         char op= ' + ';        int res=0;         int n=s.length ();        stack< Integer> stack=new stack<> ();         for (int i =0;i<n;i++) {            if (s.charAt (i) >= ' 0 '  && s.charat (i) <= ' 9 ')                  digt=digt*10+s.charat (i)-' 0 ';                 int temp=digt;             if (S.charat (i) = = ' + '  | |  s.charat (i) = = '-'  | |  s.charat (i) = = ' * '  | |  s.charat (i) = = '/'  | |  i==n-1) {                 Switch (OP) {                     case  ' + ': Stack.push (digt);break;                     case  '-': Stack.push (-digt);                      case  ' * ': Stack.push (Stack.pop () *digt);break;                     case  '/': Stack.push (Stack.pop ()/ digt); break;                }                       op=s.charat (i);                 digt=0;            }         }        while (!stack.empty ()) {             res+=stack.pop ();         }        return res;     }}`

Ps:

Stack

1. Define a temp stack to hold the operand of the operation (including the intermediate operation value)

2. D to save the last operand

3. Op to save the last operand, not the current operand

4. Traversing strings

1. If it is an operator

1. +: Depositing +d stack

2. -: Save-D to  < Span style= "margin:0px;padding:0px;" >stack

3. *: Stack The top value and the multiply of D accumulate into the stack

4. /: Stack Top Value in addition to D is deposited stack

The spatial complexity is O (n), and the time complexity is O (n). It is important to note that each time the operand is in the stack is the next operator, a bit similar to the delay operation, so the most recent operand and operator need to be stored in two additional variables. And a little trick, if we add a ' + ' operator to the front and back of the string, we can reduce the complexity of the code. such as "1+2*3" modified to "+1+2*3+".

Leetcode 227. Basic Calculator II Java language

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