题目描述
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:
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| ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
|
代码
后缀表达式，即逆波兰式计算规则为：从左到右遍历表达式的每个数字和符号，遇到数字就进栈，遇到符号，就将处于栈顶两个数字出栈，进行运算，运算结果进栈，一直到最终获得结果。
这里没有考虑不合法的表达式，代码如下：
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| public class {
public int evalRPN(String[] tokens) {
int a, b;
Stack<Integer> stack = new Stack<>();
for(String token : tokens){
if("+".equals(token)){
stack.push(stack.pop() + stack.pop());
}else if("-".equals(token)){
stack.push(-stack.pop() + stack.pop());
}else if("*".equals(token)){
stack.push(stack.pop() * stack.pop());
}else if("/".equals(token)){
a = stack.pop();
b = stack.pop();
stack.push(b / a);
}else{
stack.push(Integer.parseInt(token));
}
}
return stack.pop();
}
}
|
