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
        Stack<Integer> numStack = new Stack<>();
        
        Stack<Character> opStack = new Stack<>();
        
        opStack.push('#');
        
        s += '#';
        
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            
            // 处理数字
            if(c >='0' && c <= '9') {
                // 第一个数字直接入栈
                if (i == 0) {
                    int num = c - '0';
                    numStack.push(num);
                    continue;
                } else {
                    char pre = s.charAt(i-1);
                    // 对数字进行进位处理
                    if (pre >= '0' && pre <= '9') {
                        int num = numStack.pop() * 10 + (c - '0');
                        numStack.push(num);
                    } else {
                        int num = c - '0';
                        numStack.push(num);
                    }
                    continue;
                }
            } else {
              // 处理加减符号运算符号
                if (c == '-' || c == '+' || c == '#') {
                    // 处理负数
                    if (c == '-' && (i == 0 || s.charAt(i - 1) == '(')) {
                        numStack.push(0);
                        opStack.push('-');
                        continue;
                    }
                    // 处理单独加符号
                    if (c == '+' && (i == 0 || s.charAt(i - 1) == '(')) {
                        continue;
                    }
                    
                    // 对运算符进行运算
                    while (opStack.peek() == '*' || opStack.peek() == '+' || opStack.peek() == '-'){
                        int b = numStack.pop();
                        int a = numStack.pop();
                        char op = opStack.pop();
                        switch (op){
                            case '*':
                                numStack.push(b * a);
                                break;
                            case '+':
                                numStack.push(a + b);
                                break;
                            case '-':
                                numStack.push(a - b);
                                break;
                        }
                    }
                    opStack.push(c);
                }
                if (c == '(') {
                    opStack.push(c);
                }
                // 当前是右括号，对结果进行计算
                if (c == ')') {
                    while (opStack.peek() != '('){
                        int b = numStack.pop();
                        int a = numStack.pop();
                        char op = opStack.pop();
                        switch (op){
                            case '*':
                                numStack.push(b * a);
                                break;
                            case '+':
                                numStack.push(a + b);
                                break;
                            case '-':
                                numStack.push(a - b);
                                break;
                        }
                    }
                    // 吐出左括号
                    opStack.pop();
                }
                if (c == '*') {
                    opStack.push(c);
                }
            }
        }
        return numStack.peek();
    }
}