leetcode 逆波蘭式求解

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

Valid operators are+,-,*,/. Each operand may be an integer or another expression.

Some examples:

 ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

import
 
 java.util.Stack;
public class Solution {
     public int evalRPN(String[] tokens) {
         Stack<Integer> stack = new Stack<Integer>();
        int tmp;
        for(int i = 0; i<tokens.length; i++){
            if("+".equals(tokens[i])){
                int a = stack.pop();
                
 
int b = stack.pop();
                stack.add(b+a);
            }
            else if("-".equals(tokens[i])){
                int a = stack.pop();
                int b = stack.pop();
                stack.add(b-a);
            }
            else if("*".equals(tokens[i])){
                int a = stack.pop();
                
 
int b = stack.pop();
                stack.add(b*a);
            }
            else if("/".equals(tokens[i])){
                int a = stack.pop();
                int b = stack.pop();
                stack.add(b/a);
            }
            else{
                stack.add(Integer.parseInt(tokens[i]));
            }
            
            
        }
        return stack.pop();
     }
}

根據你波蘭式求值。看到逆波蘭式可以想到棧，掃描表達式，遇到數字則將數字入棧，遇到運算符，時，則從棧頂彈出兩個元素，後彈出的元素在運算符的左邊，先彈出的元素在元素符的右邊，執行運算，將結果入棧。掃描結束後，棧中的元素只剩下一個，即逆波蘭式的值

leetcode-Evaluate the value of an arithmetic expression in Reverse Polish Notation