题解 | #表达式求值#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 返回表达式的值
     * @param s string字符串 待计算的表达式
     * @return int整型
     */
    public int solve(String s) {
        // write code here
        /**
         * 表达式求值
         */

        //设置优先级
        HashMap<Character, Integer> map = new HashMap<>();
        map.put('+', 1);
        map.put('-', 1);
        map.put('*', 2);

        Stack<Integer> number = new Stack<>();
        Stack<Character> op = new Stack<>();
        ArrayList<Character> judge = new ArrayList<>();
        judge.add('+');
        judge.add('-');
        judge.add('*');
        judge.add('(');
        judge.add(')');

        int n = s.trim().length();

        int i = 0;
        while (i < n) {
            char c = s.charAt(i);
            if (c == '(') {
                op.push(c);
            } else if (c == ')') {
                while (!(op.peek() == '(')) {
                    //计算
                    number.push(compute(number.pop(), number.pop(), op.pop()));
                }
                op.pop();
            } else {
                if (c == '+' || c == '-' || c == '*') {
                    if (op.isEmpty() || op.peek() == '(') {
                        op.push(c);
                    }
                    //优先级高入栈
                    else if (map.get(c) > map.get(op.peek())) {
                        op.push(c);
                    } else if (map.get(c) <= map.get(op.peek())) {  //优先级低，先计算
                        //计算
                        number.push(compute(number.pop(), number.pop(), op.pop()));
                        op.push(c);
                    }
                } else {
                    int num = Integer.parseInt(String.valueOf(c));
                    if ((i + 1) < n) {
					  //循环读取完整数字
                        while ((i < n-1) && !judge.contains(s.charAt(i + 1))) {
                            num = num * 10 + Integer.parseInt(String.valueOf(s.charAt(i + 1)));
                            i++;
                        }
                    }
                    number.push(num);
                }
            }
            i++;
        }

        while (!(op.isEmpty())) {
            number.push(compute(number.pop(), number.pop(), op.pop()));
        }


        return number.peek();
    }

    public static int compute(int m, int n, char op) {
        switch (op) {
            case '+':
                return m + n;
            case '-':
                return n - m;
            case '*':
                return m * n;
        }
        return 0;
    }
}