https://leetcode-cn.com/problems/calculator-lcci/

LeetCode 16.26. 计算器

时间复杂度 O(n)

public int calculate(String s) {
        Stack<Integer> nums = new Stack<>();
        int i = 0, sum = 0;
        while (i < s.length()) {
            while (i < s.length() && s.charAt(i) == ' ') {
                i++;
            }
            if (i >= s.length()) break;
            char c = s.charAt(i);
            if (c == '*' || c == '/' || c == '+' || c == '-') {
                i++;
                while (s.charAt(i) == ' ') i++;
            }
            int num = 0;
            if (Character.isDigit(s.charAt(i))) {
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    num = num * 10 + s.charAt(i) - '0';
                    i++;
                }
            }
            switch (c) {
                case '-':
                    num = -num;
                    break;
                case '*':
                    num = nums.pop() * num;
                    break;
                case '/':
                    num = nums.pop() / num;
                    break;
            }
            nums.push(num);
        }
        while (!nums.isEmpty()) {
            sum += nums.pop();
        }
        return sum;
    }