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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
用stack, 每次遇到operator就从stack里取出2个数，后取出的数 运算符 先取出的数，然后再放回stack里。遇到数字直接放入stack里。switch不支持string，换成index。
public class Solution { public int evalRPN(String[] tokens) { Stack<String> stack=new Stack<String>(); String operators="+-*/"; for(String t:tokens){ if(!operators.contains(t)){ stack.push(t); }else{ int index=operators.indexOf(t); int b=Integer.valueOf(stack.pop()); int a=Integer.valueOf(stack.pop()); switch(index){ case 0: stack.push(String.valueOf(a+b)); break; case 1: stack.push(String.valueOf(a-b)); break; case 2: stack.push(String.valueOf(a*b)); break; case 3: stack.push(String.valueOf(a/b)); break; } } } return Integer.valueOf(stack.pop()); } }
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Evaluate Reverse Polish Notation
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原文地址：http://www.cnblogs.com/qingyezi/p/3815952.html
