LeetCode-227-基本计算器||

题目

思路

1. 直接用栈，整两个栈，最后倒一下就可以。

代码

    public int calculate(String s) {
    
        //用栈存储
        Stack<Integer> int_stack = new Stack<>();
        Stack<Character> char_stack = new Stack<>();
        for(int i=0;i<s.length();i++){
    
            //如果是空格 跳过
            Character c= s.charAt(i);
            if(c==' ') continue;
            //如果是数字 截取后面的数字
            else if(c>='0' && c<='9'){
    
                StringBuilder sb = new StringBuilder();
                sb.append(c);
                while((i+1)<s.length() && s.charAt(i+1)>='0' && s.charAt(i+1)<='9'){
    
                    sb.append(s.charAt(i+1));
                    i++;
                }
                //此刻栈顶是否为 * / 优先处理
                if(char_stack.size()>0 && (char_stack.peek()=='*' || char_stack.peek()=='/')){
    
                    //该数字同栈顶数字做算式
                    int num1 = Integer.valueOf(sb.toString());
                    int num2 = int_stack.pop();
                    char op = char_stack.pop();
                    int ans=0;
                    if(op=='*') ans = num1*num2;
                    if(op=='/') ans = num2/num1;
                    int_stack.push(ans);
                }else {
    
                    int_stack.push(Integer.valueOf(sb.toString()));
                }
            }
            //如果是算式符
            else if(c=='+' || c=='-' || c=='/' || c=='*'){
    
                //压入栈中
                char_stack.push(c);
            }
        }
        //走到这里时 char_stack只存在+ - 进行运算即可
        //此时int_stack和char应该倒过来
        Stack<Integer> int_stack_1 = new Stack<>();
        Stack<Character> char_stack_1 = new Stack<>();

        while (!int_stack.empty()) int_stack_1.push(int_stack.pop());
        while (!char_stack.empty()) char_stack_1.push(char_stack.pop());

        while(!char_stack_1.empty()){
    
            //取出char
            Character c= char_stack_1.pop();
            int num1 = int_stack_1.pop();
            int num2 = int_stack_1.pop();
            if(c=='+') int_stack_1.push(num1+num2);
            if(c=='-') int_stack_1.push(num1-num2);
        }

        return int_stack_1.peek();
    }