栈和队列（数据结构与算法）

偏执的太偏执、 2021-12-07 13:39 337阅读 0赞

* [1. 用栈实现队列](#1-用栈实现队列)
    * [2. 用队列实现栈](#2-用队列实现栈)
    * [3. 最小值栈](#3-最小值栈)
    * [4. 用栈实现括号匹配](#4-用栈实现括号匹配)
    * [5. 数组中元素与下一个比它大的元素之间的距离](#5-数组中元素与下一个比它大的元素之间的距离)
    * [6. 循环数组中比当前元素大的下一个元素](#6-循环数组中比当前元素大的下一个元素)
    
    
    
    # 1. 用栈实现队列
    
    [232. Implement Queue using Stacks (Easy)](https://leetcode.com/problems
    /implement-queue-using-stacks/description/)
    
    栈的顺序为后进先出，而队列的顺序为先进先出。使用两个栈实现队列，一个元素需要经过两个栈才能出队列，
    在经过第一个栈时元素顺序被反转，经过第二个栈时再次被反转，此时就是先进先出顺序。
    
    ```java

class MyQueue {
    
        private Stack<Integer> in = new Stack<>();
        private Stack<Integer> out = new Stack<>();
    
        public void push(int x) {
            in.push(x);
        }
    
        public int pop() {
            in2out();
            return out.pop();
        }
    
        public int peek() {
            in2out();
            return out.peek();
        }
    
        private void in2out() {
            if (out.isEmpty()) {
                while (!in.isEmpty()) {
                    out.push(in.pop());
                }
            }
        }
    
        public boolean empty() {
            return in.isEmpty() && out.isEmpty();
        }
    }

# 2. 用队列实现栈
    
    [225. Implement Stack using Queues (Easy)](https://leetcode.com/problems/
    implement-stack-using-queues/description/)
    
    在将一个元素 x 插入队列时，为了维护原来的后进先出顺序，需要让 x 插入队列首部。
    而队列的默认插入顺序是队列尾部，因此在将 x 插入队列尾部之后，需要让除了 x 之外的所有元素出队列，再入队列。

class MyStack {
    
        private Queue<Integer> queue;
    
        public MyStack() {
            queue = new LinkedList<>();
        }
    
        public void push(int x) {
            queue.add(x);
            int cnt = queue.size();
            while (cnt-- > 1) {
                queue.add(queue.poll());
            }
        }
    
        public int pop() {
            return queue.remove();
        }
    
        public int top() {
            return queue.peek();
        }
    
        public boolean empty() {
            return queue.isEmpty();
        }
    }

# 3. 最小值栈
    
    [155. Min Stack (Easy)](https://leetcode.com/problems/min-stack/description/)

class MinStack {
    
        private Stack<Integer> dataStack;
        private Stack<Integer> minStack;
        private int min;
    
        public MinStack() {
            dataStack = new Stack<>();
            minStack = new Stack<>();
            min = Integer.MAX_VALUE;
        }
    
        public void push(int x) {
            dataStack.add(x);
            min = Math.min(min, x);
            minStack.add(min);
        }
    
        public void pop() {
            dataStack.pop();
            minStack.pop();
            min = minStack.isEmpty() ? Integer.MAX_VALUE : minStack.peek();
        }
    
        public int top() {
            return dataStack.peek();
        }
    
        public int getMin() {
            return minStack.peek();
        }
    }

对于实现最小值队列问题，可以先将队列使用栈来实现，然后就将问题转换为最小值栈，这个问题出现在 编程之美：3.7。
    
    # 4. 用栈实现括号匹配
    
    [20. Valid Parentheses (Easy)](https://leetcode.com/problems/valid-parentheses/description/)
    
    ```html
    "()[]{}"
    
    Output : true
    ```

public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        for (char c : s.toCharArray()) {
            if (c == '(' || c == '{' || c == '[') {
                stack.push(c);
            } else {
                if (stack.isEmpty()) {
                    return false;
                }
                char cStack = stack.pop();
                boolean b1 = c == ')' && cStack != '(';
                boolean b2 = c == ']' && cStack != '[';
                boolean b3 = c == '}' && cStack != '{';
                if (b1 || b2 || b3) {
                    return false;
                }
            }
        }
        return stack.isEmpty();
    }

# 5. 数组中元素与下一个比它大的元素之间的距离
    
    [739. Daily Temperatures (Medium)](https://leetcode.com/problems/daily-temperatures/description/)
    
    ```html
    Input: [73, 74, 75, 71, 69, 72, 76, 73]
    Output: [1, 1, 4, 2, 1, 1, 0, 0]
    ```
    
    在遍历数组时用栈把数组中的数存起来，如果当前遍历的数比栈顶元素来的大，说明栈顶元素的下一个比它大的数就是当前元素。

public int[] dailyTemperatures(int[] temperatures) {
        int n = temperatures.length;
        int[] dist = new int[n];
        Stack<Integer> indexs = new Stack<>();
        for (int curIndex = 0; curIndex < n; curIndex++) {
            while (!indexs.isEmpty() && temperatures[curIndex] > 
                    temperatures[indexs.peek()]) {
                int preIndex = indexs.pop();
                dist[preIndex] = curIndex - preIndex;
            }
            indexs.add(curIndex);
        }
        return dist;
    }

# 6. 循环数组中比当前元素大的下一个元素
    
    [503. Next Greater Element II (Medium)](https://leetcode.com/problem
    /next-greater-element-ii/description/)
    
    ```text
    Input: [1,2,1]
    Output: [2,-1,2]
    Explanation: The first 1's next greater number is 2;
    The number 2 can't find next greater number;
    The second 1's next greater number needs to search circularly, which is also 2.
    ```
    
    与 739. Daily Temperatures (Medium) 不同的是，数组是循环数组，并且最后要求的不是距离而是下一个元素。

public int[] nextGreaterElements(int[] nums) {
        int n = nums.length;
        int[] next = new int[n];
        Arrays.fill(next, -1);
        Stack<Integer> pre = new Stack<>();
        for (int i = 0; i < n * 2; i++) {
            int num = nums[i % n];
            while (!pre.isEmpty() && nums[pre.peek()] < num) {
                next[pre.pop()] = num;
            }
            if (i < n){
                pre.push(i);
            }
        }
        return next;
    }