_227.java

package com.fishercoder.solutions;

import java.util.*;

/**
 * 227. Basic Calculator II
 *
 * 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.
 */
public class _227 {

    /**Credit: https://discuss.leetcode.com/topic/16935/share-my-java-solution*/
    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        int len = s.length();
        Deque<Integer> stack = new ArrayDeque<>();
        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.addLast(num);
                } else if (sign == '-') {
                    stack.addLast(-num);
                } else if (sign == '/') {
                    stack.addLast(stack.pollLast()/num);
                } else if (sign == '*') {
                    stack.addLast(stack.pollLast()*num);
                }
                sign = s.charAt(i);
                num = 0;
            }
        }
        int result = 0;
        while (!stack.isEmpty()) {
            result += stack.poll();
        }
        return result;
    }

}