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
        char[] chs = s.toCharArray();

        Stack<String> s0 = new Stack<>(); // 存放数字
        Stack<Character> s1 = new Stack<>(); //存放操作符
        char last = ' ';

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

            if(isChar(chs[i])){
                if(chs[i] == ')'){
                    char oper;
                    while((oper=s1.pop()) !='(') {
                        int n2 = Integer.valueOf(s0.pop());
                        int n1 = Integer.valueOf(s0.pop());
                        int res = operation(n1, n2, oper);
                        s0.push(res+"");
                    }
                } else if(s1.isEmpty() || s1.peek() =='(' || chs[i] == '(') {
                    s1.push(chs[i]);
                } else if((!s1.isEmpty() && !isPriority(chs[i], s1.peek()))) {
                    int n2 = Integer.valueOf(s0.pop());
                    int n1 = Integer.valueOf(s0.pop());
                    char oper = s1.pop();
                    int res = operation(n1, n2, oper);
                    s0.push(res+"");
                    s1.push(chs[i]);
                } else{
                    s1.push(chs[i]);
                }
            } else {
                if(!isChar(last) && !s0.isEmpty()) {
                    s0.push(s0.pop()+chs[i]);
                } else {
                    s0.push(chs[i]+"");
                }
            }
            //System.out.println(s0);
            //System.out.println(s1);
            //System.out.println("-------------------------------------");
            last = chs[i];
        }
        int res=0;
        while(!s1.empty()) {
            int n2 = Integer.valueOf(s0.pop());
            int n1 = Integer.valueOf(s0.pop());
            char oper = s1.pop();
            res = operation(n1, n2, oper);
            s0.push(res+"");
        }
        return Integer.valueOf(s0.pop());


    }

    public int getIndex(char c) {
        if(c == '+') {
            return 0;
        } else if(c == '-') {
            return 1;
        } else if(c == '*') {
            return 2;
        } else {
            return -1;
        }
    }

    public boolean isPriority(char c1, char c2) {
        boolean[][] b = {{false, false, false},{false, false, false},{true, true, false}};

        return b[getIndex(c1)][getIndex(c2)];
    }

    public boolean isChar(char c){
        return c == '+' || c == '-' || c == '*' || c == '(' || c == ')';
    }

    public int operation(int a, int b, char c){
        //System.out.println(a);
        //System.out.println(b);
        int res = Integer.MAX_VALUE;
        switch(c){
            case '+':
                res = a + b; break;
            case '-':
                res = a - b; break;
            case '*':
                res = a * b; break;
        }
        return res;
    }
}