2022-08-17 栈

剑指 Offer II 036. 后缀表达式

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class Solution {
public int evalRPN(String[] tokens) {

    // 栈的思路
    ArrayDeque<Integer> stack=new ArrayDeque<>();
    int i=0;
    while(i<tokens.length){
        if(!isNumber(tokens[i])){
            int b=stack.pollFirst();
            int a=stack.pollFirst();
           int tmp=calculate( a, b,tokens[i]);
        //    System.out.println(tmp);
           stack.addFirst(tmp);
            
        }else{
            stack.addFirst(Integer.parseInt(tokens[i]));
        }
        i++;
    }
    return stack.poll();

}
public boolean isNumber(String token) {
    return !("+".equals(token) || "-".equals(token) || "*".equals(token) || "/".equals(token));
}

public int calculate(int a,int b,String str){
    if(str.equals("+")){
        return a+b;
    }
    if(str.equals("-")){
        return a-b;
    }
    if(str.equals("*")){
        return a*b;
    }
    if(str.equals("/")){
        return a/b;
    }
    return 0;
}

}
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