227. Basic Calculator II

Topic:

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers,,, + - * , / operators and empty spaces . The integer division should truncate toward zero.

Assume that the given expression was always valid.

Some Examples:

"3+2*2" = 7 "3/2" = 1 "3+5/2" = 5

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

Links: http://leetcode.com/problems/basic-calculator-ii/

Exercises

The more concise method is to split the string according to the Operation Mr. Foo, then calculate the parts, in addition, you can also use token to do. The way to do it is tedious, which is to iterate through the string and do some special processing according to the different operators. Here are a few small places, such as the first S + + ' + ' to process the last element, save the previous operator, the result of the previous operation, and so on. The second brush must be well optimized.

Time Complexity-o (n), Space complexity-o (1)

 Public classSolution { Public intCalculate (String s) {if(s = =NULL|| S.length () = = 0)            return0; S+ = ' + '; CharPrevoperator = ' + '; intres = 0, Prevnum = 0, Curnum = 0;  for(inti = 0; I < s.length (); i++) {            Charc =S.charat (i); if(c = = "")                Continue; Else if(Character.isdigit (c)) Curnum= Curnum * 10 + (int) (C-' 0 '); Else if(c = = ' + ' | | c = = '-')) {                if(Prevoperator = = ' + ' | | prevoperator = = '-') {                    if(Prevoperator = = ' + ')) {res+ = Prevnum +Curnum; } Else{res+ = Prevnum-Curnum; }                } Else {                    if(Prevoperator = = ' * ') Res+ = Prevnum *Curnum; ElseRes+ = Prevnum/Curnum; } curnum= 0; Prevnum= 0; Prevoperator=C; } Else if(c = = ' * ' | | c = = '/')) {                if(Prevoperator = = ' + ')) Prevnum=Curnum; Else if(Prevoperator = = '-')) Prevnum= -Curnum; Else if(Prevoperator = = ' * ') Prevnum*=Curnum; Else if(Prevoperator = = '/')) Prevnum/=Curnum; Curnum= 0; Prevoperator=C; }        }                            returnRes; }}

Reference:

Https://leetcode.com/discuss/41790/10-16-lines-java-easy

Https://leetcode.com/discuss/41558/20ms-o-n-time-o-1-space-one-scan-c-solution

Https://leetcode.com/discuss/41641/17-lines-c-easy-20-ms

Https://leetcode.com/discuss/41902/share-my-java-solution

Https://leetcode.com/discuss/42903/java-straight-forward-iteration-solution-with-comments-stack

Https://leetcode.com/discuss/42423/28ms-code-with-stacks-for-oprand-extension-cover-also-given

227. Basic Calculator II