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

        s = s.trim();

        if(s == "") return 0;
        int number = 0;
        char sign = '+';

        Stack<Integer> stack = new Stack<Integer>();
        
        char[] chs = s.toCharArray();

        for(int i =0;i<chs.length;i++){

            if(Character.isDigit(chs[i])){
                number = Integer.parseInt(String.valueOf(number) + String.valueOf(chs[i]));
            }

            if(chs[i] == '('){
                int j = i+1;
                int count = 1;

                while(count>0){
                    if(chs[j] == '('){
                        count++;
                    }

                    if(chs[j] == ')'){
                        count--;
                    }

                    j++;
                }

                number = solve(s.substring(i+1,j-1));
                i = j-1;

            }

            if(!Character.isDigit(chs[i]) || i == chs.length-1){
               if(sign == '+'){
                 stack.push(number);
               }else if(sign == '-'){
                 stack.push((-1) * number);
               }else if(sign == '*'){
                 stack.push(stack.pop() * number);
               }else if(sign == '/'){
                 stack.push(stack.pop() / number); 
               }

               number = 0;
               sign = chs[i];
            }
        }

        int result = 0;
        while(!stack.isEmpty()) result += stack.pop();

        return result;

    }
}