155. Min Stack

1. Description

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 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.

You must implement a solution with O(1) time complexity for each function.

2. Example

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

3. Constraints

• -$2^{31}$ <= val <= $2^{31}$ - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * $10^{4}$ calls will be made to push, pop, top, and getMin.

4. Solutions

Stack

n is the number of nodes
Time complexity: O(n)
Space complexity: O(n)

class MinStack {
public:
    MinStack() {}

    void push(int val) {
        values.push_back(val);
        if (min_values.empty() || min_values.back() >= val) {
            min_values.push_back(val);
        }
    }

    void pop() {
        if (values.back() == min_values.back()) {
            min_values.pop_back();
        }

        values.pop_back();
    }

    int top() {
        return values.back();
    }

    int getMin() {
        return min_values.back();
    }

private:
    vector<int> values;
    vector<int> min_values;
};