227. Basic Calculator II

class Solution {
    public int calculate(String s) {
        int last = 0;
        
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == " ") {
                s = s.substring(0, i) + s.substring(i+1);
            }
        }
        
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == '/') {
                String before = s.substring(0, i - 1);
                String after = s.substring(i+2);
                
                int val = Integer.valueOf(s.charAt(i-1)) / Integer.valueOf(s.charAt(i+1));
                
                String v = Integer.toString(val);
                
                s = before + v + after;
            }
            
            if (s.charAt(i) == '*') {
                String before = s.substring(0, i - 1);
                String after = s.substring(i+2);
                
                int val = Integer.valueOf(s.charAt(i-1)) * Integer.valueOf(s.charAt(i+1));
                
                String v = Integer.toString(val);
                
                s = before + v + after;
            }
        }
        
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == '+') {
                String before = s.substring(0, i - 1);
                String after = s.substring(i+2);
                
                int val = Integer.valueOf(s.charAt(i-1)) + Integer.valueOf(s.charAt(i+1));
                
                String v = Integer.toString(val);
                
                s = before + v + after;
            }
            
            if (s.charAt(i) == ' ') {
                String before = s.substring(0, i - 1);
                String after = s.substring(i+2);
                
                int val = Integer.valueOf(s.charAt(i-1)) - Integer.valueOf(s.charAt(i+1));
                
                String v = Integer.toString(val);
                
                s = before + v + after;
            }
        }
        
        return Integer.valueOf(s);
    }
}

长く书いてあります.成功しなかったことは本当ですか.
冷静に.stackを使ったことがありますが失敗しluceneが見えました^^

class Solution {
    public int calculate(String s) {

        if (s == null || s.isEmpty()) return 0;
        int len = s.length();
        Stack<Integer> stack = new Stack<Integer>();
        int currentNumber = 0;
        char operation = '+';
        for (int i = 0; i < len; i++) {
            char currentChar = s.charAt(i);
            if (Character.isDigit(currentChar)) {
                currentNumber = (currentNumber * 10) + (currentChar - '0');
            }
            if (!Character.isDigit(currentChar) && !Character.isWhitespace(currentChar) || i == len - 1) {
                if (operation == '-') {
                    stack.push(-currentNumber);
                }
                else if (operation == '+') {
                    stack.push(currentNumber);
                }
                else if (operation == '*') {
                    stack.push(stack.pop() * currentNumber);
                }
                else if (operation == '/') {
                    stack.push(stack.pop() / currentNumber);
                }
                operation = currentChar;
                currentNumber = 0;
            }
        }
        int result = 0;
        while (!stack.isEmpty()) {
            result += stack.pop();
        }
        return result;
    }
}

Runtime: 9 ms, faster than 69.76% of Java online submissions for Basic Calculator II.
Memory Usage: 39 MB, less than 77.02% of Java online submissions for Basic Calculator II.