Leetcode Queue & Stack Problems

Queue and Stack

There are multiple implementations of Queue in Java, such as LinkedList, ArrayDeque, etc. offer, poll, peek methods are used to manipulate data in the queue. Circular array is another choice to implement an queue.

Leetcode 158. Read N Characters Given Read4 II - Call multiple times

For stack, we use Stack class and push, pop, peek methods.

Leetcode 225. Implement Stack using Queues

Leetcode 232. Implement Queue using Stacks

Monotonic stack

Monotonic stack is used to find the next lager element in an array. Vice verser.

Stack<Integer> stack = new Stack<>();
Map<Integer, Integer> nextGreaterMap = new HashMap();

for(int i = 0; i < nums2.length; i++){
  while(!stack.isEmpty() && stack.peek() < nums2[i]){
    nextGreaterMap.put(stack.pop(), nums2[i]); //pop all element smaller than the current one.
  }
  stack.push(nums2[i]);
}

Leetcode 42. Trapping Rain Water

Leetcode 84. Largest Rectangle in Histogram

Leetcode 316. Remove Duplicate Letters

Leetcode 496. Next Greater Element I

Leetcode 739. Daily Temperatures