Leetcode 150. Evaluate Reverse Polish Notation------java

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators is + , - , * , / . Each operand is an integer or another expression.

Some Examples:

  ["2", "1", "+", "3", "*")--((2 + 1) (3)-9  ["4", "", "5", "/", "+"], 4 + (13/5))

is to reverse the result of the Polish expression (subsequent traversal).

1, direct solution, very slow

 Public classSolution { Public intEVALRPN (string[] tokens) {intLen =tokens.length; if(len = = 0)            return0;  for(inti = 0; i < Len; i + + ){                        if(Tokens[i].equals ("+") Helper (Tokens,i,1); Else if(Tokens[i].equals ("-") Helper (Tokens,i,2); Else if(Tokens[i].equals ("*") Helper (Tokens,i,3); Else if(Tokens[i].equals ("/") Helper (Tokens,i,4); }        returnInteger.valueof (tokens[0]); }     Public voidHelper (string[] tokens,intPosinttype) {        intPOS1 = -1,pos2 = 0 ; Tokens[pos]=NULL;  while(POS >= 0 ){            if(Tokens[pos]! =NULL){                if(POS1 = =-1) POS1=POS; Else{Pos2=POS;  Break; }} POS--; }        intNUM1 =integer.valueof (TOKENS[POS1]); intnum2 =integer.valueof (Tokens[pos2]); if(Type = = 1) {Tokens[pos2]= String.valueof (num1+num2); }Else if(Type = = 2) {Tokens[pos2]= String.valueof (num2-NUM1); }Else if(Type = = 3) {Tokens[pos2]= String.valueof (num2*NUM1); }Else if(Type = = 4) {Tokens[pos2]= String.valueof (num2/NUM1); } TOKENS[POS1]=NULL; }     }

2, the use of stacks, very simple.

 Public classSolution { Public intEVALRPN (string[] tokens) {intLen =tokens.length; if(len = = 0)            return0; Stack<Integer> stack =NewStack<integer>();  for(inti = 0; i < Len; i + + ){                        if(Tokens[i].equals ("+") ){                intNUM1 =Stack.pop (); intnum2 =Stack.pop (); Stack.push (NUM1+num2); }            Else if(Tokens[i].equals ("-") ){                intNUM1 =Stack.pop (); intnum2 =Stack.pop (); Stack.push (num2-NUM1); }            Else if(Tokens[i].equals ("*") ){                intNUM1 =Stack.pop (); intnum2 =Stack.pop (); Stack.push (NUM1*num2); }            Else if(Tokens[i].equals ("/") ){                intNUM1 =Stack.pop (); intnum2 =Stack.pop (); Stack.push (num2/+NUM1); }Else{Stack.push (integer.valueof (tokens[i])); }        }        returnStack.pop (); }        }

Leetcode 150. Evaluate Reverse Polish Notation------java