【leetcode】224. Basic Calculator

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Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

Input: " 2-1 + 2 "
Output: 3

Example 3:

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Note:

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

题解如下：

class Solution{
    public static int calculate(String s) {
	    int n = s.length();
        int sign = 1;
        int res = 0;
        Stack<Integer> stack = new Stack<>();
        char[] a = s.toCharArray();
        for(int i = 0;i < n;i++) {
            if(Character.isDigit(a[i])) {
                int sum = a[i] - '0';
                while(i + 1 < n && Character.isDigit(a[i+1])) {
                    sum = sum * 10 + a[i + 1] -'0';
                    i++;
                }
                res += sum * sign;
            } else if(a[i] == '+') {
                sign = 1;
            } else if(a[i] == '-') {
                sign = -1;
            } else if(a[i] == '(') {
                stack.push(res);
                stack.push(sign);
                res = 0;
                sign = 1;
            } else if(a[i] == ')') {
                res = res * stack.pop() + stack.pop();
            }
        }
        return res;
    }
}