D100_Get Min from Stack.cpp

// Given q queries, You task is to implement the following four functions for a stack:

// push(x) – Insert an integer x onto the stack.
// pop() – Remove the top element from the stack.
// peek() - Return the top element from the stack. If the stack is empty, return -1.
// getMin() – Retrieve the minimum element from the stack in O(1) time. If the stack is empty, return -1.
// Each query can be one of the following:

// 1 x : Push x onto the stack.
// 2 : Pop the top element from the stack.
// 3: Return the top element from the stack. If the stack is empty, return -1.
// 4: Return the minimum element from the stack.
// Examples:

// Input: q = 7, queries = [[1, 2], [1, 3], [3], [2], [4], [1, 1], [4]]
// Output: [3, 2, 1]
// Explanation:
// push(2): Stack is [2]
// push(3): Stack is [2, 3]
// peek(): Top element is 3
// pop(): Removes 3, stack is [2]
// getMin(): Minimum element is 2
// push(1): Stack is [2, 1]
// getMin(): Minimum element is 1
// Input: q = 4, queries = [[1, 4], [1, 2], [4], [3]]
// Output: [2, 2]
// Explanation:
// push(4): Stack is [4]
// push(2): Stack is [4, 2]
// getMin(): Minimum element is 2
// peek(): Top element is 2
// Input: q = 5, queries = [[1, 10], [4], [1, 5], [4], [2]]
// Output: [10, 5]
// Explanation:
// push(10): Stack is [10]
// getMin(): Minimum element is 10
// push(5): Stack is [10, 5]
// getMin(): Minimum element is 5
// pop(): Removes 5, stack is [10]

class Solution
{
public:
    int mini;
    stack<int> st;
    Solution()
    {
    }
    void push(int x)
    {
        if (st.empty())
            mini = x;
        else if (x <= mini)
        {
            st.push(mini);
            mini = x;
        }
        st.push(x);
    }
    void pop()
    {
        if (st.empty())
            return;
        int temp = st.top();
        st.pop();
        if (temp == mini && !st.empty())
        {
            mini = st.top();
            st.pop();
        }
    }
    int peek()
    {
        if (st.empty())
            return -1;
        return st.top();
    }
    int getMin()
    {
        if (st.empty())
            return -1;
        return mini;
    }
};