16-Reverse-First-K-Elements-of-Queue.md

Reverse First K Elements of Queue

Problem Link

Practice Problem

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Pattern

• Queue
• Recursion

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Approach

Remove the first k elements with recursion or a stack, then reinsert them in reversed order.

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Time Complexity

O(n)

Space Complexity

O(k)

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Java Solution

import java.util.*;
class Solution {
    public Queue<Integer> reverseFirstK(Queue<Integer> queue, int k) {
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < k; i++) stack.push(queue.poll());
        while (!stack.isEmpty()) queue.offer(stack.pop());
        int size = queue.size();
        for (int i = 0; i < size - k; i++) queue.offer(queue.poll());
        return queue;
    }
}