commit

Author: Shreya111111

Stack/Basic Calculator/BasicCalculator.java

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+class Solution {
+    public int calculate(String s) {
+   Stack<Integer> stack = new Stack<Integer>();
+   int result = 0;
+   int number = 0;
+   int sign = 1;
+   for(int i = 0; i < s.length(); i++){
+       char c = s.charAt(i);
+       if(Character.isDigit(c)){
+           number = 10 * number + (int)(c - '0');
+       }else if(c == '+'){
+           result += sign * number;
+           number = 0;
+           sign = 1;
+       }else if(c == '-'){
+           result += sign * number;
+           number = 0;
+           sign = -1;
+       }else if(c == '('){
+           //we push the result first, then sign;
+           stack.push(result);
+           stack.push(sign);
+           //reset the sign and result for the value in the parenthesis
+           sign = 1;   
+           result = 0;
+       }else if(c == ')'){
+           result += sign * number;  
+           number = 0;
+           result *= stack.pop();    //stack.pop() is the sign before the parenthesis
+           result += stack.pop();   //stack.pop() now is the result calculated before the parenthesis
+           
+       }
+   }
+   if(number != 0) result += sign * number;
+   return result;
+}   
+   }

Stack/Basic Calculator/BasicCalculator.md

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+# Basic Calculator
+Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
+
+Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
+```
+ Example 1:
+
+Input: s = "1 + 1"
+Output: 2
+Example 2:
+
+Input: s = " 2-1 + 2 "
+Output: 3
+Example 3:
+
+Input: s = "(1+(4+5+2)-3)+(6+8)"
+Output: 23
+ 
+Constraints:
+
+1 <= s.length <= 3 * 105
+s consists of digits, '+', '-', '(', ')', and ' '.
+s represents a valid expression.
+'+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).
+'-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).
+There will be no two consecutive operators in the input.
+Every number and running calculation will fit in a signed 32-bit integer.