Topic

• Stack
• Design

Description

https://leetcode.com/problems/min-stack/

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• getMin() – Retrieve the minimum element in the stack.

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]Output
[null,null,null,null,-3,null,0,-2]Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

Constraints:

• Methods pop, top and getMin operations will always be called on non-empty stacks.

Analysis

底层用单链表实现

Submission

public class MinStack {/** initialize your data structure here. */private ListNode headNode;private ListNode minNode;public MinStack() {}public void push(int x) {headNode = new ListNode(x, headNode);minNode = minNode == null ? new ListNode(x) //: new ListNode(x < minNode.val ? x : minNode.val, minNode);}public void pop() {if (headNode == null) {return;} else {ListNode p = headNode;headNode = p.next;p.next = null;p = minNode;minNode = p.next;p.next = null;}}public int top() {if (headNode == null)throw new NullPointerException("Stack is Empty.");return headNode.val;}public int getMin() {if (minNode == null)throw new NullPointerException("Stack is Empty.");return minNode.val;}private class ListNode {public int val;public ListNode next;public ListNode(int val) {this.val = val;}ListNode(int val, ListNode next) {this.val = val;this.next = next;}}
}

Test

import static org.junit.Assert.*;import org.junit.Test;public class MinStackTest {@Testpublic void test() {MinStack minStack = new MinStack();minStack.push(-2);minStack.push(0);minStack.push(-3);assertEquals(-3, minStack.getMin());minStack.pop();assertEquals(0, minStack.top());assertEquals(-2, minStack.getMin());}
}