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
        int ret = 0;
        int[] num = new int[s.length()];
        int nu = 0;
        char[] symbol = new char[s.length()];
        int sy = 0; 

        for (int i = 0; i < s.length(); i++){
            if (s.charAt(i) == '+' || s.charAt(i) == '-' || s.charAt(i) == '*' || s.charAt(i) == '(' || s.charAt(i) == ')'){
                
                if (sy == 0){
                    symbol[sy++] = s.charAt(i);
                        
                }else{
                    
                    switch(s.charAt(i)){
                        case '+':
                        case '-':
                        if (symbol[sy-1] == s.charAt(i) || symbol[sy-1] == '*'){
                            //出栈
                            switch(symbol[sy-1]){
                                case '+':
                                num[nu-2] = num[nu-2] + num[nu-1];
                                nu--;
                                sy--;
                                break;
                                case '-':
                                num[nu-2] = num[nu-2] - num[nu-1];
                                nu--;
                                sy--;
                                break;
                                case '*':
                                num[nu-2] = num[nu-2] * num[nu-1];
                                nu--;
                                sy--;
                                break;
                            }
                            //入栈
                            symbol[sy++] = s.charAt(i);
                        }else{
                            //入栈
                            symbol[sy++] = s.charAt(i);
                        }
                        break;
                        case '*':
                        if (symbol[sy-1] == s.charAt(i)){
                            //出栈
                            num[nu-2] = num[nu-2] * num[nu-1];
                            nu--;
                            sy--;
                            //入栈
                            symbol[sy++] = s.charAt(i);
                        }else{
                            symbol[sy++] = s.charAt(i);
                        }
                        break;
                        case '(':
                        symbol[sy++] = s.charAt(i);
                        break;
                        case ')':
                        //出栈
                        while(symbol[sy-1] != '('){
                            switch(symbol[sy-1]){
                                case '+':
                                num[nu-2] = num[nu-2] + num[nu-1];
                                nu--;
                                sy--;
                                break;
                                case '-':
                                num[nu-2] = num[nu-2] - num[nu-1];
                                nu--;
                                sy--;
                                break;
                                case '*':
                                num[nu-2] = num[nu-2] * num[nu-1];
                                nu--;
                                sy--;
                                break;
                            }
                        }
                        sy--;
                        break;
                    }
                
                }
            }else{
                int tmp = s.charAt(i) - '0';
                while((i+1) < s.length() && s.charAt(i + 1) <= '9' && s.charAt(i + 1) >= '0'){
                    
                    tmp *= 10;
                    tmp += s.charAt(++i) - '0';
                }
                
                num[nu++] = tmp;
                

            }
        }
        
        while(sy != 0){
            switch(symbol[sy-1]){
                case '+':
                num[nu-2] = num[nu-2] + num[nu-1];
                nu--;
                sy--;
                break;
                case '-':
                    

                num[nu-2] = num[nu-2] - num[nu-1];
                nu--;
                sy--;
                break;
                case '*':
                num[nu-2] = num[nu-2] * num[nu-1];
                nu--;
                sy--;
                break;
            }
        }
        
        ret = num[0];
        return ret;
    }
}