Basic Calculator-优快云博客

本文链接：https://blog.youkuaiyun.com/iteye_6755/article/details/82642718

本文介绍了一种不使用内置eval函数的简易计算器实现方法，通过解析字符串表达式并利用栈来完成加减运算及括号内表达式的计算。

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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 .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

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

public class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        int res = 0;
        int sign = 1;
        int num = 0;
        for (int i = 0; i < s.length(); i++) {
			char ch = s.charAt(i);
			if (ch <= '9' && ch >= '0') {
				num = 10 * num + ch - '0';
			} else if (ch == '+') {
				res += sign * num;
				sign = 1;
				num = 0;
			} else if (ch == '-') {
				res += sign * num;
				sign = -1;
				num = 0;
			} else if (ch == '(') {
				stack.push(res);
				stack.push(sign);
				res = 0;
				sign = 1;
			}  else if (ch == ')') {
				res += sign * num;
				num = 0;
				res *= stack.pop();
				res += stack.pop();
			}
		}
        if (num != 0) {
        	res += sign * num;
        }
        return res;
    }
}