[LeetCode] Problem 227 - Basic Calculator II

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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

No.1

Input: “3+2*2”

Output: 7

No.2

Input: “ 3/2 “

Output: 1

No.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.

Code

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public int calculate(String s) {
    int result = 0;
    int curResult = 0;
    int num = 0;
    char lastSign = '+';

    for (int i = 0; i < s.length(); i++) {
        char ch = s.charAt(i);

        if (Character.isDigit(ch)) {
            num = 10 * num + (ch - '0');
        }

        if (ch == '+' || ch == '-' || ch == '*' || ch == '/' || i == s.length() - 1) {
            switch (lastSign) {
                case '+': curResult += num; break;
                case '-': curResult -= num; break;
                case '*': curResult *= num; break;
                case '/': curResult /= num; break;
            }

            if (ch == '+' || ch == '-' || i == s.length() - 1) {
                result += curResult;
                curResult = 0;
            }

            lastSign = ch;
            num = 0;
        }
    }

    return result;
}