155. Min Stack

Prompt

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.

Examples

• 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

In C++

class MinStack {
private:
    stack<pair<int, int>> backing;
    int min_val = INT_MAX;
public:
    MinStack() {}
    
    void push(int val) {
        min_val = min(min_val, val);
        backing.push(make_pair(val, min_val));
    }
    
    void pop() {
        backing.pop();
        min_val = getMin();
    }
    
    int top() {
        if (backing.empty()) return INT_MAX;
        return backing.top().first;
    }
    
    int getMin() {
        if (backing.empty()) return INT_MAX;
        return backing.top().second;
    }
};

Explanation

The idea is that for each stack value you also store the minimum of the stack at the point that that value was pushed. The other details aren’t really important.