155. Min Stack

https://leetcode.com/problems/min-stack/

Problem

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.

• pop() -- Removes the element on top of the stack.

• top() -- Get the top element.

• getMin() -- Retrieve 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:

• Methods pop, top and getMin operations will always be called on non-empty stacks.

Solution

class MinStack {
    // first = element, second = min_element
    stack<pair<int, int>> min_stack_;
public:
    /** initialize your data structure here. */
    MinStack() {
        
    }
    
    void push(int x) {
        const int min_element = (min_stack_.empty()) ? x : min(min_stack_.top().second, x);
        min_stack_.push({x, min_element});
    }
    
    void pop() {
        min_stack_.pop();
    }
    
    int top() {
        return min_stack_.top().first;
    }
    
    int getMin() {
        return min_stack_.top().second;
    }
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(x);
 * obj->pop();
 * int param_3 = obj->top();
 * int param_4 = obj->getMin();
 */

• #stack

• #implementation