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.

Example 1:

Input: "3+2*2"
Output: 7

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

Input: " 3+5 / 2 "
Output: 5

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

Approach #1: Stack. [Java]

class Solution {
    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        Stack<Integer> stack = new Stack();
        s += '+';
        char op = '+';
        for (int i = 0, n = 0; i < s.length(); ++i) {
            char c = s.charAt(i);
            if (c >= '0' && c <= '9') { n = n * 10 + c - '0'; continue; }
            if (c == ' ') continue;
            if (op == '+') stack.push(n);
            else if (op == '-') stack.push(-n);
            else if (op == '*') stack.push(stack.pop() * n);
            else if (op == '/') stack.push(stack.pop() / n);
            op = c;
            n = 0;
        }
        
        int ret = 0;
        while (!stack.empty()) 
            ret += stack.pop();
        
        return ret;
    }
}