"Leetcode" 224. Basic Calculator

Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ) , the plus + or minus sign - , Non-negati ve integers and empty spaces .

Assume that the given expression was always valid.

Some Examples:

"1 + 1" = 2 "2-1 + 2" = 3 "(1+ (4+5+2)-3) + (6+8)" = 23

Note:do not use the eval built-in library function.

Two points:

1, without parentheses, sequential execution

2, in parentheses, first execute the parentheses in the

Two stacks:

A storage operand, each time the stack to note that if the operation Fu Yi the top element is ' + '/'-', it needs to be calculated immediately.

A storage operator (including parentheses), each time ') ', is constantly doing a stack calculation and then into the stack until the ' (', indicating that the current parentheses are calculated.

classSolution { Public:    intCalculatestrings) {stack<int>num; Stack<int>op; inti =0;  while(I <s.size ()) {             while(I < S.size () && s[i] = =' ') I++; if(i = =s.size ()) Break; if(S[i] = ='+'|| S[i] = ='-'|| S[i] = ='(') {Op.push (s[i]); I++; }            Else if(S[i] = =')')            {                 while(Op.top ()! ='(')                {//calculation within parentheses                    intN2 =Num.top ();                    Num.pop (); intN1 =Num.top ();                    Num.pop (); if(Op.top () = ='+') Num.push (N1+n2); ElseNum.push (N1-n2);                Op.pop ();                } op.pop ();  while(!op.empty () && op.top ()! ='(')                {                    intN2 =Num.top ();                    Num.pop (); intN1 =Num.top ();                    Num.pop (); if(Op.top () = ='+') Num.push (N1+n2); ElseNum.push (N1-n2);                Op.pop (); } I++; }            Else            {                intn =0;  while(I < S.size () && S[i] >='0'&& S[i] <='9') {n= NTen+ (s[i]-'0'); I++;                } num.push (n);  while(!op.empty () && op.top ()! ='(')                {                    intN2 =Num.top ();                    Num.pop (); intN1 =Num.top ();                    Num.pop (); if(Op.top () = ='+') Num.push (N1+n2); ElseNum.push (N1-n2);                Op.pop (); }            }        }        returnNum.top (); }};

"Leetcode" 224. Basic Calculator