155. Min Stack

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:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   -->Returns -3.
minStack.pop();
minStack.top();      -->Returns 0.
minStack.getMin();   --> Returns -2.

S: two stack

用额外的stack维护当前最小值，当push进的值小于minStk.top()时push进minStk

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {

    }

    void push(int x) {
        stk.push(x);
        if (minStk.empty() || x <= minStk.top()) {
            minStk.push(x);
        }
    }

    void pop() {
        if (stk.empty()) return;
        if (!minStk.empty() && stk.top() == minStk.top()) {
            minStk.pop();
        }
        stk.pop();
    }

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

    int getMin() {
        return minStk.empty() ? stk.top() : minStk.top();
    }
private:
    stack<int> stk;
    stack<int> minStk;
};