227. Basic Calculator II

http://www.cnblogs.com/grandyang/p/4601208.html

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.
Example 1:
Input: "3+2*2"
Output: 7
Example 2:
Input: " 3/2 "
Output: 1
Example 3:
Input: " 3+5 / 2 "
Output: 5











Use stack to solve this question 


Solution 1 : use two stacks 


不是我写的， 有些地方还不懂

class Solution {
    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        Stack<Integer> nums = new Stack<>(); // the stack that stores numbers
        Stack<Character> ops = new Stack<>(); // the stack that stores operators (including parentheses)
        int num = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == ' ') continue;
            if (Character.isDigit(c)) {
                num = c - '0';
                // iteratively calculate each number
                while (i < s.length() - 1 && Character.isDigit(s.charAt(i+1))) {
                    num = num * 10 + (s.charAt(i+1) - '0');
                    i++;
                }
                nums.push(num);
                num = 0; // reset the number to 0 before next calculation
            } else if (c == '(') {
                ops.push(c);
            } else if (c == ')') {
                // do the math when we encounter a ')' until '('
                while (ops.peek() != '(') nums.push(operation(ops.pop(), nums.pop(), nums.pop()));
                ops.pop(); // get rid of '(' in the ops stack
                // ？ 不知道这个precedence 在做什么？ 
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                while (!ops.isEmpty() && precedence(c, ops.peek())) nums.push(operation(ops.pop(), nums.pop(),nums.pop()));
                ops.push(c);
            }
        }
        while (!ops.isEmpty()) {
            nums.push(operation(ops.pop(), nums.pop(), nums.pop()));
        }
        return nums.pop();
    }

    private static int operation(char op, int b, int a) {
        switch (op) {
            case '+': return a + b;
            case '-': return a - b;
            case '*': return a * b;
            case '/': return a / b; // assume b is not 0
        }
        return 0;
    }
    // helper function to check precedence of current operator and the uppermost operator in the ops stack 
    // 这个 helper function， 不懂在做什么
    private static boolean precedence(char op1, char op2) {
        if (op2 == '(' || op2 == ')') return false;
        if ((op1 == '*' || op1 == '/') && (op2 == '+' || op2 == '-')) return false;
        return true;
    }
}







Solution 2:  without using stack 


class Solution {
    public int calculate(String s) {
      // remove all empty space 
      s = s.replaceAll(" ", "");
      int sum = s.charAt(0) - '0';
      int prev = s.charAt(0) - '0';
      int index = 0;
      char op = '+';

      while(index + 1 < s.length() && Character.isDigit(s.charAt(index+1))){
        sum = sum * 10 + (s.charAt(index + 1) - '0');
        prev = prev * 10 + (s.charAt(index + 1) - '0');
        index++;
      }

    
      
      for(int i = index + 1; i < s.length(); i++){
        char c = s.charAt(i);
        if(!Character.isDigit(c)){
          op = c;
        }else{
          int current = c - '0';
          while(i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))){
            current = current * 10 + (s.charAt(i + 1) - '0');
            i++;
          }
          if(op == '+'){
            sum = sum + current;
            prev = current;
          }
          if(op == '-'){
            sum = sum - current;
            prev = -current;
          }
          if(op == '*'){
            sum = sum - prev + prev * current;
            prev = prev * current;
          }
          if(op == '/'){
            sum = sum - prev + prev / current;
            prev = prev / current;
          }
        }
      }
      return sum;
    }
}