150. Evaluate Reverse Polish Notation

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<Integer>();
        for(int i = 0; i < tokens.length; i++){
            switch(tokens[i]){
                case "+":
                    stack.push(stack.pop() + stack.pop());
                    break;
                case "-":
                    stack.push(-stack.pop() + stack.pop());
                    break;
                case "*":
                    stack.push(stack.pop() * stack.pop());
                    break;
                case "/":
                    int n1 = stack.pop(), n2 = stack.pop();
                    stack.push(n2 / n1);
                    break;
                default:
                    stack.push(Integer.parseInt(tokens[i]));    
            }
        }
        return stack.pop();
    }
}

71. Simplify Path

class Solution {
    public String simplifyPath(String path) {
        if(path == null || path.length() == 0) return "";
        path = path.trim();
        Stack<String> stack = new Stack<>();
        String[] s = path.split("/");
        
        for(int i = 0; i < s.length; i++){
            //..
            if(!stack.isEmpty() && s[i].equals("..")){
                stack.pop();
            }else if(!s[i].equals(".") && !s[i].equals("..") && !s[i].equals("")){//.,..,空白以外的
                stack.push(s[i]);    
            }
        }
        //组装
        List<String> list = new ArrayList<>(stack);
        return "/" + String.join("/", list);
    }
}

 

388. Longest Absolute File Path

最后len -1是减去末尾不需要的 / 的长度。

class Solution {
   public int lengthLongestPath(String input) {
        Deque<Integer> stack = new ArrayDeque<>();
        stack.push(0); // "dummy" length
        int maxLen = 0;
        for(String s:input.split("\n")){
            int lev = s.lastIndexOf("\t")+1; // number of "\t"
            while(lev+1<stack.size()) stack.pop(); // find parent
            int len = stack.peek()+s.length()-lev+1; // remove "/t", add"/"
            stack.push(len);
            // check if it is file
            if(s.contains(".")) maxLen = Math.max(maxLen, len-1); 
        }
        return maxLen;
    }
}