Day 10 | 232. 用栈实现队列 | 225. 用队列实现栈 | 20. 有效的括号 | 1047. 删除字符串中的所有相邻重复项

232. 用栈实现队列 - 力扣（LeetCode）

import java.util.Stack;

public class MyQueue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    public void push(int x) {
        stack1.push(x);
    }

    public int pop() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }

    public int peek() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.peek();
    }

    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

225. 用队列实现栈 - 力扣（LeetCode）

class MyStack {
   private Queue<Integer> q1;
   private Queue<Integer> q2;

   public MyStack() {
       q1 = new LinkedList<>();
       q2 = new LinkedList<>();
   }

   public void push(Integer x) {
       q1.offer(x);
   }

   public int pop() {
       while (q1.size() > 1) {
           q2.offer(q1.poll());
       }
       int topElement = q1.poll();
       Queue<Integer> temp = q1;
       q1 = q2;
       q2 = temp;
       return topElement;
   }

   public int top() {
       while (q1.size() > 1) {
           q2.offer(q1.poll());
       }
       int topElement = q1.poll();
       q2.offer(topElement);
       Queue<Integer> temp = q1;
       q1 = q2;
       q2 = temp;
       return topElement;
   }

   public boolean empty() {
       return q1.isEmpty();
   }
}

/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/

20. 有效的括号 - 力扣（LeetCode）

class Solution {

   private static final Map<Character, Character> map = new HashMap<Character, Character>() {
       {
           put('{', '}');
           put('[', ']');
           put('(', ')');
           put('?', '?');
       }
   };

   public boolean isValid(String s) {
       if (s.length() > 0 && !map.containsKey(s.charAt(0)))
           return false;
       LinkedList<Character> stack = new LinkedList<Character>() {
           {
               add('?');
           }
       };
       for (Character c : s.toCharArray()) {
           if (map.containsKey(c))
               stack.addLast(c);
           else if (map.get(stack.removeLast()) != c)
               return false;
       }
       return stack.size() == 1;
   }
}

1047. 删除字符串中的所有相邻重复项 - 力扣（LeetCode）

class Solution {
   public String removeDuplicates(String s) {
       StringBuffer stack = new StringBuffer();
       int top = -1;
       for (int i = 0; i < s.length(); i++) {
           char ch = s.charAt(i);
           if (top >= 0 && stack.charAt(top) == ch) {
               stack.deleteCharAt(top);
               --top;
           } else {
               stack.append(ch);
               ++top;
           }
       }
       return stack.toString();
   }
}

加油。