Leetcode 224 Basic Calculator, leetcodecalculator

Leetcode 224 Basic Calculator, leetcodecalculator
1. Problem Description

Calculates the value of a string expression. The expression only contains (,), +,-, spaces, and non-negative integers. For example:
"1 + 1" = 2
"2-1 + 2" = 3
"(1 + (4 + 5 + 2)-3) + (6 + 8)" = 23
Link: https://leetcode.com/problems/basic-calculator/

2. methods and ideas 2.1 using suffix expressions for Computation

One idea is to convert the infix expression to the suffix expression for calculation according to the conventional method. The conversion process is as follows:
Traverse the string in sequence and perform the following operations.

class Solution {public:    int GetNum(string poststr,int *i)    {        int tmp =0;        while(poststr[(*i)] >= '0' && poststr[(*i)] <= '9')        {            tmp = tmp*10 + ( poststr[(*i)] - '0');            (*i) ++;        }        return tmp;    }    int postcal(string poststr)    {        stack<int> s;        int i,a,b,tmp =0;        for(i=0; i < poststr.length(); i++)        {            switch(poststr[i])            {                case ' ':                    continue;                case '+':                    b = s.top(); s.pop();                    a = s.top(); s.pop();                    s.push(a+b);                    break;                case '-':                    b = s.top(); s.pop();                    a = s.top(); s.pop();                    s.push(a-b);                    break;                default:                    s.push(GetNum(poststr,&i));            }        }        return s.top();    }    int calculate(string s) {        int re =0,tmp = 0;        stack<char> num;        string poststr="";        for(int i=0; i < s.length(); i++)        {            switch(s[i])            {                case '(':                    num.push(s[i]);                     break;                case ' ':                    continue;                case ')':                    poststr += " ";                    while(num.top() != '(')                    {                        poststr += num.top();                        num.pop();                    }                    num.pop();                    break;                case '+':                case '-':                    poststr += " ";                    while(!num.empty() && num.top() != '(')                    {                        poststr +=num.top();                        num.pop();                    }                    num.push(s[i]);                    break;                default:                    poststr += s[i];            }            }        poststr += " ";        while(!num.empty())        {            poststr += num.top();            num.pop();        }        //cout<<poststr<<endl;        return postcal(poststr);     }};

However, this method times out and is dizzy. You have to take a good look and see if there are any optimizations !!!

2.2 Expression simplification

Since the expression only contains parentheses and addition and subtraction operations, we can simplify the expression by adding and subtraction rules and then evaluate the value.
　　

class Solution {public:    int calculate(string s) {        stack<int> num;        num.push(1);        char op = '+';                long re = 0;        for (int i=0; i<s.length(); i++) {            switch(s[i]) {                case ' ':                    break;                case '+':                case '-':                    op = s[i];                    break;                case '(':                    num.push(num.top() * (op == '-' ? -1 : 1));                    op = '+';                    break;                case ')':                    num.pop();                    break;                default:                    int tmp = 0;                    while (  s[i] >= '0' && s[i] <= '9') {                        tmp = tmp * 10 + s[i] - '0';                        i++;                    }                    i--;                    re += (op == '-'?-1:1)*num.top()*tmp;            }        }        return re;    }};