https://leetcode.com/problems/basic-calculator/

basic-calculator.java

/**
Solution One: Recursion Solution
**/
class Solution {
    public int calculate(String s) {
        return cal(s, new int[]{0});
    }
    
    private int cal(String s, int[] p) {
        int res = 0;
        int num = 0;
        int sign = 1;
        while(p[0] < s.length()) {
            if (Character.isDigit(s.charAt(p[0]))) {
                num = num * 10 + s.charAt(p[0]) - '0';
                p[0] ++;
            } else if (s.charAt(p[0]) == '+') {
                res += (sign * num);
                num = 0;
                sign = 1;
                p[0] ++;
            } else if (s.charAt(p[0]) == '-') {
                res += (sign * num);
                num = 0;
                sign = -1;
                p[0] ++;
            } else if (s.charAt(p[0]) == '(') {
                p[0] ++; // skip (
                int val = cal(s, p);
                res += (sign * val);
            } else if (s.charAt(p[0]) == ')') {
                p[0] ++;
                res += (sign * num);
                return res;
            } else {
                p[0] ++;
            }
        }
        res += (sign * num);
        
        return res;
    }
}

/**
Solution Two
Non Recursion 
**/

class Solution {
    public int calculate(String s) {
        
        int res = 0;
        int num = 0;
        int sign = 1;
        
        int p = 0;
        Deque<Integer> stack = new ArrayDeque<>();
        while(p < s.length()) {
            if (Character.isDigit(s.charAt(p))) {
                num = num * 10 + s.charAt(p) - '0';
                p++;
            } else if (s.charAt(p) == '+') {
                res += (sign * num);
                num = 0;
                sign = 1;
                p++;
            } else if (s.charAt(p) == '-') {
                res += (sign * num);
                num = 0;
                sign = -1;
                p++;
            } else if (s.charAt(p) == '(') {
                p++;
                stack.addLast(res);
                stack.addLast(sign);
                res = 0;
                num = 0;
                sign = 1;
            } else if (s.charAt(p) == ')') {
                p++;
                res += (sign * num);
                int preSign = stack.pollLast();
                int preRes = stack.pollLast();
                num = 0;
                sign = 1;
                res = preSign * res + preRes;
            } else {
                p++;
            }
        }
        res += (sign * num);
        return res;
    }    
}

/**
Solution Three
May support multiply and divie, really slow
**/
class Solution {
    public int calculate(String s) {
        Deque<Integer> stack = new ArrayDeque<>();
        Deque<Character> ops = new ArrayDeque<>();
        
        int idx = 0;
        int len = s.length();
        boolean seeNum = false;
        int num = 0;
        while(idx < len) {
            char c = s.charAt(idx);
            if (Character.isDigit(c)) {
                seeNum = true;
                num = num * 10 + c - '0';
            } else if ( c == '+' || c == '-' || c == ')') {
                if (stack.isEmpty()) {
                    stack.addLast(num);
                } else if (!ops.isEmpty() && ops.peekLast() == '(') {
                    if (seeNum) {
                        stack.addLast(num);
                    }
                } else if (!ops.isEmpty()) {
                    char op = ops.pollLast();
                    int preNum = stack.pollLast();
                    stack.addLast(cal(preNum, num, op));
                }
                if (c == ')') {
                    ops.pollLast(); // remove (
                    if (!ops.isEmpty() && ops.peekLast() != '(') {
                        char op = ops.pollLast();
                        int num2 = stack.pollLast();
                        int num1 = stack.pollLast();
                        stack.addLast(cal(num1, num2, op));
                    }
                } else {
                    ops.addLast(c);
                }
                num = 0;
                seeNum = false;
            } else if (c == '(') {
                ops.addLast(c);
            }
            idx += 1;
        }
        if (!ops.isEmpty() && ops.peekLast() != '(') {
            char op = ops.pollLast();
            int preNum = stack.pollLast();
            stack.addLast(cal(preNum, num, op));
        }
        return stack.isEmpty() ? num : stack.pollLast();
    }
    
    private int cal(int num1, int num2, char op) {
        System.out.println(num1 + " " + op + " " + num2);
        if (op == '+') {
            return num1 + num2;
        } else if (op == '-') {
            return num1 - num2;
        }
        return 0;
    }
}