227. Basic Calculator II

Leetcode String

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.

分析

```java public static int calculate(String s) { if (s == null || s.length() == 0) return -1; int current = 0; int result = 0; Stack stack = new Stack<>(); char sign = '+'; while (current < s.length()) { char x = s.charAt(current); while (x == ' ') { // 数字前有空格 current++; x = s.charAt(current); } int integer = 0; while (x >= '0' && x <= '9') { integer = integer * 10 + x - '0'; current++; if (current >= s.length()) break; x = s.charAt(current); } if (sign == '*') { stack.push(stack.pop() * integer); } else if (sign == '/') { stack.push(stack.pop() / integer); } else if (sign == '-') { stack.push(-integer); } else if (sign == '+') { stack.push(integer); } // 数字后有空格 while (current < s.length() && s.charAt(current) == ' ') { current++; } if (current < s.length()) sign = s.charAt(current); current++; }

    for (int p : stack) {
        result += p;
    }
    return result;
}
```