题解 | #表达式求值#

import java.util.; import java.io.; public class Main{ public static void main(String[] args) throws Exception { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); // 优先级maps HashMap<Character, Integer> maps = new HashMap<>(); maps.put('+', 1); maps.put('-', 1); maps.put('*', 2); maps.put('/', 2); maps.put('%', 2); maps.put('^', 3); String str = ""; while ((str = br.readLine()) != null) { // 双栈法 // 数据栈 Deque nums = new LinkedList<>(); nums.push(0); // 操作栈 Deque ops = new LinkedList<>(); // 预处理 str = str.replaceAll(" ", ""); str = str.replaceAll("\(-", "\(0-"); str = str.replaceAll("\(+", "\(0+"); if (str.charAt(0) == '-') { // 特判首字符是负数 str = '0' + str; } // 转换成字符数组 char[] chars = str.toCharArray(); int n = str.length(); for(int i = 0; i < n; i++) { if (chars[i] == '(') { ops.push(chars[i]); } else if (chars[i] == ')') { while(!ops.isEmpty()) { if (ops.peek() == '(') { ops.pop(); break; } else { calc(nums, ops); } }

            } else {
                if (isNumber(chars[i])) {
                   // 当前遇到的是数字
                    int j = 0;
                    int k = i;
                    while(k < n && isNumber(chars[k])) {
                        j = j * 10 + (chars[k] - '0');
                        k++;
                    }
                    nums.push(j);
                    i = k - 1;
                } else {
                    // 当前遇到的是加减乘除
                    while (!ops.isEmpty() && ops.peek() != '(') {
                        if (maps.get(ops.peek()) >= maps.get(chars[i])) {
                            calc(nums, ops);
                        } else {
                            break;
                        }
                    }
                    ops.push(chars[i]);
                }
            }
        }
        while (!ops.isEmpty() && ops.peek() != '(') {
            calc(nums, ops);
        }
        System.out.println(nums.peek());
    }
}

// 判断是否是数字
public static boolean isNumber(char cur) {
    return Character.isDigit(cur);
}

 //  进行计算
public static void calc(Deque<Integer> nums, Deque<Character> ops) {
    if (ops.isEmpty()) return;
    if (nums.isEmpty() || nums.size() < 2) return;
    int b = nums.pop();
    int a = nums.pop();
    int op = ops.pop();
    int res = 0;
    if (op == '+') {
        res = a + b;
    } else if (op == '-') {
        res = a - b;
    } else if (op == '*') {
        res = a * b;
    } else if (op == '/') {
        res = a / b;
    } else if (op == '^') {
        res = (int)Math.pow(a, b);
    } else if (op == '%') {
        res = a % b;
    }
    nums.push(res);
}

}