basicCalculatorII.java

// 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 A STACK and keep track of sign
TC: O(N) where N is length of string
SC: O(N) where N is length of string

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

    int re = 0;
    while(!stack.isEmpty()){
        re += s.pop();
    }
    return re;
}
}

OPTIMIZED without the stack, use a "LAST NUMBER" to represent the stack
TC: O(N)
SC: O(1)
class Solution {
    public int calculate(String s) {
        if (s == null || s.isEmpty()) return 0;
        int length = s.length();
        int currentNumber = 0, lastNumber = 0, result = 0;
        char operation = '+';
        for (int i = 0; i < length; i++) {
            char currentChar = s.charAt(i);
            if (Character.isDigit(currentChar)) {
                currentNumber = (currentNumber * 10) + (currentChar - '0');
            }
            if (!Character.isDigit(currentChar) && !Character.isWhitespace(currentChar) || i == length - 1) {
                if (operation == '+' || operation == '-') {
                    result += lastNumber;
                    lastNumber = (operation == '+') ? currentNumber : -currentNumber;
                } else if (operation == '*') {
                    lastNumber = lastNumber * currentNumber;
                } else if (operation == '/') {
                    lastNumber = lastNumber / currentNumber;
                }
                operation = currentChar;
                currentNumber = 0;
            }
        }
        result += lastNumber;
        return result;
    }
}