解析算术表达式

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解析算术表达式的思路

算术表达式转换为后缀表达式
计算后缀表达式的值
1. 如果是操作数，则压入栈中
2. 如果是操作符，则从栈中取出两个值，进行运算，把运算之后的值，压入栈中，直到栈为空结束

Java代码

package StackAndQueue;import java.util.Scanner;/** * Created by ln on 2017/5/14. */public class Postfix3 {    public static void main(String[] args){        Scanner s = new Scanner(System.in);        String ss = s.nextLine();        String love = getValue(ss);        System.out.println(love);        int value = getR(love);        System.out.println(value);    }    //后缀表达式求值    public static int getR(String s){        Stack stack = new Stack(s.length());        char v;        int i1;        int i2;        int value;        for(int i=0;i<s.length();i++){            v = s.charAt(i);            if(v>='1'&&v<='9'){                stack.push(Integer.parseInt(v+""));            }else {                i1 = stack.pop();                i2 = stack.pop();                value = getYS(v,i2,i1);                stack.push(value);            }        }        return stack.pop();    }    //转换为后缀表达式    public static String getValue(String s){        Stack stack = new Stack(s.length());        String r = "";        char v;        int v_p;        char top;        int top_p;        for(int i=0;i<s.length();i++){            v = s.charAt(i);            if(v>='1'&&v<'9'){                r=r+v;            }else {                if(stack.isEmpty()){                    stack.push(v);                }else {                    v_p = getL(v);                    top = (char)stack.pop();                    top_p=getL(top);                    if(top_p==3||v=='('||(top_p==1&&v_p==2)){                        stack.push(top);                        stack.push(v);                    }else if(v==')'){                        while (top!='('){                            r=r+top;                            top=(char)stack.pop();                        }                    }else if(top_p==v_p){                        r=r+top;                        stack.push(v);                    }else {                        while (top!='('){                            r=r+top;                            if(stack.isEmpty()){                                break;                            }                            top = (char)stack.pop();                        }                        stack.push(v);                    }                }            }        }        while (!stack.isEmpty()){            top = (char)stack.pop();            r=r+top;        }        return r;    }    //    public static int getL(char c){        int v = -1;        switch (c){            case '+':            case '-':v=1;break;            case '*':            case '/':v=2;break;            case '(':            case ')':v=3;break;        }        return v;    }    //    public static int getYS(char c,int i1,int i2){        int v = 0;        switch (c){            case '+':v=i1+i2;break;            case '-':v=i1-i2;break;            case '*':v=i1*i2;break;            case '/':v=i1/i2;break;        }        return v;    }}

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