概述

对Leetcode中的基本计算器的总结

只能计算加减的

public static int cal(String str){
        Stack<Integer> stack=new Stack<>();
        String sign="+";
        int curnum=0;
        boolean hasNum=false;
        for(int i=0;i<str.length();i++)
        {
            char temp=str.charAt(i);
            if(Character.isDigit(temp))
            {
                curnum=curnum*10+(temp-'0');
            }else if(temp!=' ')
            {
                if(sign.equals("+"))
                {
                    stack.push(curnum);
                }else if(sign.equals("-"))
                {
                    stack.push(curnum*-1);
                }
                sign=String.valueOf(temp);
                curnum=0;
            }
        }
        if(sign.equals("+"))
        {
            stack.push(curnum);
        }else if(sign.equals("-"))
        {
            stack.push(curnum*-1);
        }
        return 0;
    }

加减乘除，不带括号

import java.util.Stack;
class Solution {

    public int calculate(String str)
    {
        Stack<Integer> stack=new Stack<>();
        int curnum=0;
        String sign="+";
        for(int i=0;i<str.length();i++)
        {
            char temp=str.charAt(i);
            if(Character.isDigit(temp))
            {
                curnum=curnum*10+temp-'0';
            }
            if((!Character.isDigit(temp)&&temp!=' ')||i==str.length()-1)
            {
                if(sign.equals("+"))
                {
                    stack.push(curnum);
                }else if(sign.equals("-"))
                {
                    stack.push(curnum*-1);
                }
                else if(sign.equals("*"))
                {
                    int t=stack.pop();
                    curnum=t*curnum;
                    stack.push(curnum);
                }
                else if(sign.equals("/"))
                {
                    int t=stack.pop();
                    curnum=t/curnum;
                    stack.push(curnum);
                }
                curnum=0;
                sign=String.valueOf(temp);
            }
        }
        int res=0;
        for(Integer i:stack)
        {
            res+=i;            
        }
        return res;
    }


    public static int calculate2(String s) {
        Stack<String> stack=new Stack<>();
        char[] chars = s.toCharArray();
        int curnum=0;
        boolean hasNum=false;
        for(int i=0;i<chars.length;i++)
        {
            if(chars[i]>='0'&&chars[i]<='9')
            {
                curnum=curnum*10+chars[i]-'0';
                hasNum=true;
            }else if(chars[i]!=' ')
            {
                if(hasNum)
                {
                    if(!stack.isEmpty()&&(stack.peek().equals("*")||stack.peek().equals("/")))
                    {
                        String point=stack.pop();
                        int tempnum=Integer.parseInt(stack.pop());
                        if(point.equals("*"))
                        {
                            stack.push(String.valueOf(tempnum*curnum));
                        }else
                        {
                            stack.push(String.valueOf(tempnum/curnum));
                        }
                    }else
                    {
                        stack.push(String.valueOf(curnum));
                    }
                    curnum=0;
                    hasNum=false;
                }
                stack.push(String.valueOf(chars[i]));
            }
        }
        if(hasNum)
        {
            if(!stack.isEmpty()&&(stack.peek().equals("*")||stack.peek().equals("/")))
            {
                String point=stack.pop();
                int tempnum=Integer.parseInt(stack.pop());
                if(point.equals("*"))
                {
                    stack.push(String.valueOf(tempnum*curnum));
                }else
                {
                    stack.push(String.valueOf(tempnum/curnum));
                }
            }else
            {
                stack.push(String.valueOf(curnum));
            }
        }
        int res=0;
        boolean flag=true;
        for(String str:stack)
        {
             if(str.equals("+"))
            {
                flag=true;
            }else if(str.equals("-"))
            {
                flag=false;
            }else
            {
               if(flag) res+=Integer.parseInt(str);
               else res-=Integer.parseInt(str);
            }
        }
        return res;
    }
}

完整的代码

 public static int calculate(String str)
    {
        Stack<Integer> stack=new Stack<>();
        int[] res = cal(str, 0);
        return res[0];
    }
    public static int[] cal(String str,int index)
    {
        Stack<Integer> stack=new Stack<>();
        int curnum=0;
        String sign="+";
        int i=index;
        for(;i<str.length();i++)
        {
            char temp=str.charAt(i);
            if(Character.isDigit(temp))
            {
                curnum=curnum*10+temp-'0';
            }
            if((!Character.isDigit(temp)&&temp!=' ')||i==str.length()-1)
            {
                if(temp=='(')
                {
                    int[] arr=cal(str,i+1);
                    curnum=arr[0];
                    if(curnum==-4)
                        System.out.println(curnum);
                    i=arr[1]+1;
                }
                if(sign.equals("+"))
                {
                    stack.push(curnum);
                }else if(sign.equals("-"))
                {
                    stack.push(curnum*-1);
                }
                else if(sign.equals("*"))
                {
                    int t=stack.pop();
                    curnum=t*curnum;
                    stack.push(curnum);
                }
                else if(sign.equals("/"))
                {
                    int t=stack.pop();
                    curnum=t/curnum;
                    stack.push(curnum);
                }
                if(i==str.length()-1||temp==')')
                    break;
                curnum=0;
                sign=String.valueOf(temp);
            }
        }
        int res=0;
        for(Integer t:stack)
        {
            res+=t;
        }
        return new int[]{res,i};
    }