155. Min Stack

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155. Min Stack

Description

Difficulty: Medium

Related Topics: Stack, Design

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.

Example 1:

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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, top and getMin operations will always be called on non-empty stacks.
  • At most 3 * 104 calls will be made to push, pop, top, and getMin.

Hints/Notes

Solution

Language: C++

Use one stack of pairs

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class MinStack {
public:
    stack<pair<int, int>> stk;

    MinStack() {

    }

    void push(int val) {
        stk.emplace(val, min(val, getMin()));
    }

    void pop() {
        stk.pop();
    }

    int top() {
        return stk.top().first;
    }

    int getMin() {
        return stk.empty() ? INT_MAX : stk.top().second;
    }
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(val);
 * obj->pop();
 * int param_3 = obj->top();
 * int param_4 = obj->getMin();
 */
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class MinStack {
public:
    int min;
    stack<int> stk;
    stack<int> minStk;

    MinStack() {
        min = INT_MAX;
    }
    
    void push(int val) {
        stk.push(val);
        if (val <= min) {
            min = val;
            minStk.push(min);
        }
    }
    
    void pop() {
        if (stk.top() == min) {
            minStk.pop();
            min = minStk.empty() ? INT_MAX : minStk.top();
        }
        stk.pop();
    }
    
    int top() {
        return stk.top();
    }
    
    int getMin() {
        return min;
    }
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(val);
 * obj->pop();