Min Stack

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(val) pushes the element val onto 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

Solution:

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class MinStack {
private List<Integer> values = null;
private List<Integer> mins = null;

public MinStack() {
this.values = new ArrayList<Integer>();
this.mins = new ArrayList<Integer>();
}

private void setMin(int val) {
if(this.mins.isEmpty() || val <= this.getMin()) {
this.mins.add(val);
}
}

public void push(int val) {
this.values.add(val);
setMin(val);
}

public void pop() {
if(this.values.remove(this.values.size() - 1) == this.getMin()) {
this.mins.remove(this.mins.size() - 1);
}
}

public int top() {
return this.values.get(this.values.size() - 1);
}

public int getMin() {
return this.mins.get(this.mins.size() - 1);
}
}