Easy
Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
Implement the MinStack class:
MinStack()initializes the stack object.void push(int val)pushes the elementvalonto the stack.void pop()removes the element on the top of the stack.int top()gets the top element of the stack.int getMin()retrieves 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:
-231 <= val <= 231 - 1- Methods
pop,topandgetMinoperations will always be called on non-empty stacks. - At most
3 * 104calls will be made topush,pop,top, andgetMin.
class Node {
public $min;
public $data;
public $nextNode;
public $previousNode;
function __construct($min, $data, $previousNode, $nextNode) {
$this->min = $min;
$this->data = $data;
$this->previousNode = $previousNode;
$this->nextNode = $nextNode;
}
}
class MinStack {
private $currentNode;
function __construct() {
// no initialization needed.
}
/**
* @param Integer $val
* @return NULL
*/
public function push($val) {
if ($this->currentNode == null) {
$this->currentNode = new Node($val, $val, null, null);
} else {
$this->currentNode->nextNode = new Node(min($this->currentNode->min, $val), $val, $this->currentNode, null);
$this->currentNode = $this->currentNode->nextNode;
}
}
/**
* @return NULL
*/
public function pop() {
$this->currentNode = $this->currentNode->previousNode;
}
/**
* @return Integer
*/
public function top() {
return $this->currentNode->data;
}
/**
* @return Integer
*/
function getMin() {
return $this->currentNode->min;
}
}