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
        Map<Character,Integer> map = new HashMap<>();
        map.put('+',1);
        map.put('-',1);
        map.put('*',2);
        Deque<Integer> nums = new ArrayDeque<>();
        Deque<Character> opts = new ArrayDeque<>();
        s.replaceAll(" ","");
        char c1[] = s.toCharArray();
        int n = s.length();

        for(int i=0;i<n;i++){
            char c = c1[i];
            if(isnumber(c)){
                int num=0;
                int j=i;
                while(j<n && isnumber(c1[j]))
                    num = num*10+(c1[j++]-'0');
                nums.push(num);
                i=j-1;
            }
            else if(c=='(')
                opts.push(c);
            else if(c==')'){
                while(!opts.isEmpty()&&opts.peek()!='(')
                    cal(nums,opts);
                opts.pop();
            } 
            else{
                while(!opts.isEmpty()&& opts.peek()!='(' && map.get(c)<=map.get(opts.peek())){
                    cal(nums,opts);
                }
                opts.push(c);
            }
        }
        while(!opts.isEmpty()){
            cal(nums,opts);
        }
        return nums.pop();
    }
    public static boolean isnumber(Character c){
        return Character.isDigit(c);
    }
    public static void cal(Deque<Integer> nums, Deque<Character> opts){
        int num2 = nums.pop();
        int num1 = nums.pop();
        char opt = opts.pop();
        if(opt=='+')
            nums.push(num1+num2);
        else if(opt=='-')
            nums.push(num1-num2);
        else    
            nums.push(num1*num2);
    }
}