LeetCode:Basic Calculator II

Basic Calculator II

Total Accepted: 23197  Total Submissions: 91345  Difficulty: Medium

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.

You may assume that the given expression is always valid.

Some examples:

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

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

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

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  (H) Basic Calculator (H) Expression Add Operators

java code:

public class Solution {
    public int calculate(String s) {
        int len = s.length();
        
        if(s==null || len==0) return 0;
        
        Stack<Integer> stack = new Stack<Integer>();
        
        char sign = '+';
        int num = 0;
        for(int i=0;i<len;i++) {
            char c = s.charAt(i);
            if(Character.isDigit(c))
                num = 10 * num + c-'0';
                
            if(!Character.isDigit(c) && c != ' ' || i==len-1) {
                switch(sign) {
                    case '+':
                        stack.push(num);
                        break;
                    case '-':
                        stack.push(-num);
                        break;
                    case '*':
                        stack.push(stack.pop()*num);
                        break;
                    case '/':
                        stack.push(stack.pop()/num);
                }
                sign = c;
                num = 0;
            }
        }
        
        int ans = 0;
        for(int x:stack)
            ans += x;
        return ans;
    }
}